Chapter Four
ELECTROMAGNETIC FORCES IN ACTION

5. Electromagnetic waves in nature

5-1. Sun rays

“The sticky leaves that bloom in spring are dear to me, the blue sky is dear to me,” said Ivan Karamazov, one of the heroes born of the genius of Dostoevsky.

Sunlight has always been and remains for a person a symbol of eternal youth, all the best that can be in life. The excited joy of a man living under the Sun is felt in the first poem of a four-year-old boy:

May the sun always be
May there always be sky, May there always be mother,
May I always be!

And in the quatrain of the wonderful poet Dmitry Kedrin:

You say our fire is out.
You say that we have grown old with you,
Look how blue the sky is!

The dark kingdom, the kingdom of darkness, is not just the absence of light, but a symbol of everything heavy, oppressing the human soul.

Sun worship is the oldest and most beautiful cult of mankind. This is the fabulous god Kon-Tiki of the Peruvians, this is the deity of the ancient Egyptians - Ra. At the very dawn of their existence, people were able to understand that the Sun is life. We have known for a long time that the Sun is not a deity, but a hot ball, but the reverent attitude towards it will remain with humanity forever.

Even a physicist accustomed to dealing with accurate registration of phenomena feels as if he is committing blasphemy when he says that sunlight is electromagnetic waves of a certain length and nothing more. But this is exactly so, and we should try to talk only about this in our book.

As light, we perceive electromagnetic waves with a wavelength of 0.4 micrometers to 0.72 micrometers (and if the red light is very bright, then up to 0.8 micrometers or a little more). Other waves do not cause visual impressions.

The wavelength of light is very short. Imagine an average sea wave that has increased so much that it occupies the entire Atlantic Ocean from New York in America to Lisbon in Europe. The wavelength of light at the same magnification would only slightly exceed the width of a book page.

5-2. Gas and electromagnetic waves

But we know perfectly well that there are electromagnetic waves of a completely different wavelength. There are kilometer waves; there are also shorter than visible light: ultraviolet, X-rays, etc. Why did nature make our eye (as well as the eyes of animals) sensitive to a certain, relatively narrow, wavelength range?

On the scale of electromagnetic waves, visible light occupies a tiny band sandwiched between ultraviolet and infrared rays. Along the edges are broad bands of radio waves and gamma rays emitted by atomic nuclei.

All these waves carry energy, and it would seem that they could just as well do for us what light does. The eye might be sensitive to them.

Of course, one can immediately say that not all wavelengths are suitable. Gamma rays and X-rays are emitted noticeably only under special circumstances, and they are almost non-existent around us. Yes, and thank God. They (this applies especially to gamma rays) cause radiation sickness, so that humanity could not enjoy the picture of the world in gamma rays for a long time.

Long radio waves would be extremely inconvenient. They freely go around meter-sized objects, just as sea waves go around protruding coastal stones, and we could not examine objects that are clearly vital for us to see. Waves bending around obstacles (diffraction) would lead to the fact that we would see the world "like a fish in the mud."

But there are also infrared (thermal) rays that can heat bodies, but are invisible to us. It would seem that they could successfully replace those wavelengths that the eye perceives. Or, finally, the eye could adjust to the ultraviolet.

Well, the choice of a narrow strip of wavelengths, which we call visible light, in this section of the scale, is completely random? The sun emits both visible light and ultraviolet and infrared rays.

No and no! This is far from the case. First of all, the maximum radiation of electromagnetic waves by the Sun lies just in the yellow-green region of the visible spectrum. But this is not the main thing! The emission will also be quite intense in neighboring regions of the spectrum.

5-3. "Windows" in the atmosphere

We live at the bottom of the air ocean. The earth is surrounded by an atmosphere. We consider it transparent or almost transparent. And she

is such in reality, but only for a narrow section of wavelengths (a narrow section of the spectrum, as physicists say in such a case), which our eye perceives.

This is the first, optical "window" in the atmosphere. Oxygen strongly absorbs ultraviolet light. Water vapor blocks infrared radiation. Long radio waves are thrown back, reflected from the ionosphere.

There is only one more "radio window", transparent for waves from 0.25 centimeters to about 30 meters. But these waves, as already mentioned, are poorly suited for the eye, and their intensity in the solar spectrum is very low. It took a great leap in the development of radio technology, caused by the improvement of radar during the Second World War, to learn how to reliably pick up these waves.

Thus, in the process of the struggle for existence, living organisms acquired an organ that reacted precisely to those radiations that were most intense and very well suited for their purpose.

The fact that the maximum radiation of the Sun exactly falls in the middle of the "optical window" should probably be considered an additional gift from nature. (Nature in general turned out to be exceptionally generous towards our planet. We can say that she did everything, or almost everything, in her power so that we could be born and live happily. She, of course, could not “foresee” all the consequences of her generosity, but it gave us reason and thereby made us responsible for our own future fate.) The amazing coincidence of the maximum radiation of the Sun with the maximum transparency of the atmosphere could probably be dispensed with. The rays of the Sun would sooner or later awaken life on Earth anyway and would be able to support it in the future.

5-4. Blue sky

If you are reading this book not as a manual for self-education, which is a pity to abandon, because time and money have already been spent, but “with feeling, sense, arrangement”, then you should pay attention to an obvious, it would seem, contradiction. The maximum radiation of the Sun falls on the yellow-green part of the spectrum, and we see it as yellow.

Blame the atmosphere. It transmits the long-wave part of the spectrum (yellow) better and the short-wave part is worse. Therefore, the green light is greatly weakened.

Short wavelengths are generally scattered in all directions by the atmosphere with particular intensity. Therefore, above us “the sky is shining blue”, and not yellow or red. If there were no atmosphere at all, there would be no habitual sky above us. Instead, there is a black abyss with a dazzling Sun. So far, only astronauts have seen it.

Such a Sun without protective clothing is fatal. High in the mountains, when there is still something to breathe, the Sun becomes unbearably burning *): you can not stay without clothes, and in the snow - without dark glasses. You can burn the skin and retina of the eyes.

*) Ultraviolet radiation is insufficiently absorbed by the upper layers of the atmosphere.

Note SuperCook. The main source of the blueness of the earth's sky is the oxygen of the atmosphere (nitrogen is colorless). Dust in the air dissipates this blueness of oxygen, making it whitish. The cleaner the air, the brighter and bluer the sky. If Earth had an atmosphere of chlorine, the sky would be green.

5-5. Gifts of the Sun

Light waves falling on the Earth are a priceless gift of nature. First of all, they give warmth, and with it life. Without them, the cosmic cold would have shackled the Earth. If the amount of all the energy consumed by mankind (fuel, falling water and wind) increased 30 times, then even then it would be only a thousandth of the energy that the Sun supplies us free of charge and without any hassle.

In addition, the main types of fuel - coal and oil - are nothing but "canned sunbeams." These are the remains of vegetation that once covered our planet with a lush color, and perhaps, in part, of the animal world.

The water in the turbines of power plants was once in the form of steam lifted up by the energy of the sun's rays. It is the sun's rays that move the air masses in our atmosphere.

But that is not all. Light waves not only heat. They awaken a chemical activity in the substance that simple heating cannot cause. Fabric fading and tanning are the result of chemical reactions.

The most important reactions take place in the "sticky spring leaves", as well as, however, in the needles of needles, leaves of grass, trees, and in many microorganisms. In a green leaf under the Sun, the processes necessary for all life on Earth take place. They give us food, they also give us oxygen to breathe.

Our body, like the organisms of other higher animals, is not able to combine pure chemical elements into complex chains of atoms - molecules of organic substances. Our breath continuously poisons the atmosphere. When we consume vital oxygen, we exhale carbon dioxide (CO2), bind oxygen and make the air unbreathable. It needs to be cleaned continuously. This is done for us by plants on land and microorganisms in the oceans.

Leaves absorb carbon dioxide from the air and break it down into its constituent parts: carbon and oxygen. Carbon is used to build living plant tissues, and pure oxygen is returned to the air. Attaching to the carbon chain the atoms of other elements extracted by their roots from the earth, plants build molecules of proteins, fats and carbohydrates: food for us and for animals.

All this happens due to the energy of the sun's rays. And here, not only the energy itself is especially important, but the form in which it comes. Photosynthesis (as scientists call this process) can proceed only under the influence of electromagnetic waves in a certain range of the spectrum.

We will not attempt to explain the mechanism of photosynthesis. It has not yet been fully elucidated. When this happens, a new era will probably begin for humanity. Proteins and other organic substances can be grown directly in retorts under a blue sky.

5-6. light pressure

Light generates the finest chemical reactions. At the same time, he is capable of simple mechanical actions. He presses on the surrounding bodies. True, even here the light shows a certain delicacy. The light pressure is very low. On a square meter of the earth's surface on a clear sunny day, there is only about half a milligram of force.

A fairly significant force acts on the entire globe, about 60,000 tons, but it is negligible compared to the gravitational force (1014 times less).

Therefore, the enormous talent of PN Lebedev was needed to detect light pressure. At the beginning of our century, he measured the pressure not only on solids, but also on gases.

Despite the fact that the light pressure is very small, its effect can sometimes be observed directly with the naked eye. To do this, you need to see a comet.

It has long been noted that the tail of a comet, consisting of the smallest particles, when it moves around the Sun, is always directed in the opposite direction from the Sun.

The particles of a comet's tail are so small that the forces of light pressure turn out to be comparable or even superior to the forces of their attraction to the Sun. Therefore, comet tails are repelled from the Sun.

It is not difficult to understand why this is happening. The force of gravity is proportional to the mass and, therefore, the cube of the linear dimensions of the body. The solar pressure is proportional to the size of the surface and, therefore, to the square of the linear dimensions. As the particles decrease, the gravitational forces consequently decrease faster than the pressure, and for sufficiently small particle sizes they become less strength light pressure.

An interesting incident occurred with the American satellite "Echo". After the satellite entered orbit, a large polyethylene shell was filled with compressed gas. A light ball with a diameter of about 30 meters was formed. Unexpectedly, it turned out that in one revolution by the pressure of the sun's rays, it is displaced from orbit by 5 meters. As a result, instead of 20 years, as planned, the satellite stayed in orbit for less than a year.

Inside the stars, at a temperature of several million degrees, the pressure of electromagnetic waves must reach an enormous value. It must be assumed that, along with gravitational forces and ordinary pressure, it plays an essential role in intrastellar processes.

The mechanism of the appearance of light pressure is comparatively simple, and we can say a few words about it. The electric field of an electromagnetic wave incident on a substance shakes the electrons. They begin to oscillate in the transverse direction to the direction of wave propagation. But this in itself does not cause pressure.

The magnetic field of the wave begins to act on the electrons that have moved. It just pushes the electrons along the light beam, which ultimately leads to the appearance of pressure on a piece of matter as a whole.

5-7. Heralds of distant worlds

We know how great the boundless expanses of the Universe are, in which our Galaxy is an ordinary cluster of stars, and the Sun is a typical star belonging to the number of yellow dwarfs. Only within the solar system is the privileged position of the globe revealed. Earth is the most habitable of all the planets in the solar system.

We know not only the location of countless stellar worlds, but also their composition. They are built from the same atoms as our Earth. The world is one.

Light is the messenger of distant worlds. He is the source of life, he is also the source of our knowledge of the universe. “How great and beautiful the world is,” the electromagnetic waves coming to Earth tell us. Only electromagnetic waves "speak" - gravitational fields do not give any equivalent information about the Universe.

Stars and star clusters can be seen with the naked eye or through a telescope. But how do we know what they are made of? Here the spectral apparatus comes to the aid of the eye, "sorting" light waves according to their lengths and sending them out in different directions.

Heated solids or liquids emit a continuous spectrum, i.e., all possible wavelengths, ranging from long infrared to short ultraviolet.

A completely different matter is isolated or almost isolated atoms of incandescent vapors of a substance. Their spectrum is a palisade of colored lines of different brightness, separated by wide dark stripes. Each colored line corresponds to an electromagnetic wave of a certain length *).

*) Note, by the way, that outside of us in nature there are no colors, there are only waves of different lengths.

Most importantly, the atoms of any chemical element give their own spectrum, unlike the spectra of atoms of other elements. Like human fingerprints, the line spectra of atoms have a unique personality. The uniqueness of the patterns on the skin of the finger helps to find the criminal. In the same way, the individuality of the spectrum makes it possible for physicists to determine the chemical composition of a body without touching it, and not only when it lies nearby, but also when it is removed at distances that even light travels in millions of years. It is only necessary that the body glows brightly **).

**) The chemical composition of the Sun and stars is determined, strictly speaking, not from the emission spectra, for this is a continuous spectrum of the dense photosphere, but from the absorption spectra of the Sun's atmosphere. Vapors of a substance absorb most intensively precisely those wavelengths that they emit in a hot state. The dark absorption lines against the background of the continuous spectrum make it possible to establish the composition of the celestial bodies.

Those elements that are on Earth were also "found" on the Sun and stars. Helium was even earlier discovered on the Sun and then found on Earth.

If the radiating atoms are in a magnetic field, then their spectrum changes significantly. Separate colored stripes are split into several lines. This is what makes it possible to detect the magnetic field of stars and estimate its magnitude.

The stars are so far away that we cannot directly see whether they are moving or not. But the light waves coming from them bring us this information as well. The dependence of the wavelength on the speed of the source (the Doppler effect, which was already mentioned earlier) makes it possible to judge not only the speeds of stars, but also their rotation.

The main information about the universe comes to us through the "optical window" in the atmosphere. With the development of radio astronomy, more and more new information about the Galaxy comes through the "radio window".

5-8. Where do electromagnetic waves come from

SuperCook Note: The only source of electromagnetic waves is the acceleration of charged particles. And such accelerations can occur for completely different reasons.

We know, or think we know, how radio waves are born in the universe. One of the sources of radiation was mentioned earlier in passing: thermal radiation arising from the deceleration of colliding charged particles. Of greater interest is non-thermal radio emission.

Visible light, infrared and ultraviolet rays are almost exclusively thermal in origin. Heat The sun and other stars are the main cause of the birth of electromagnetic waves. Stars also emit radio waves and X-rays, but their intensity is very low.

When charged particles of cosmic rays collide with atoms of the earth's atmosphere, short-wave radiation is born: gamma and x-rays. True, being born in the upper layers of the atmosphere, they are almost completely absorbed, passing through its thickness, and do not reach the surface of the Earth.

The radioactive decay of atomic nuclei is the main supplier of gamma rays near the Earth's surface. Here, energy is drawn from the richest "energy storeroom" of nature - the atomic nucleus.

Radiate electromagnetic waves and all living beings. First of all, like any heated body, infrared rays. Individual insects (such as fireflies) and deep-sea fish emit visible light. Here it is born due to chemical reactions in the luminous organs (cold light).

Finally, during chemical reactions associated with cell division of plant and animal tissues, ultraviolet radiation is emitted. These are the so-called mitogenetic rays, discovered by the Soviet scientist Gurvich. At one time it seemed that they had great importance in the vital activity of cells, but later more accurate experiments, as far as one can judge, gave rise to a number of doubts here.

5-9. Sense of smell and electromagnetic waves

It cannot be said that only visible light acts on the sense organs. If you put your hand to a hot kettle or stove, you will feel heat in the distance, our body is able to perceive fairly intense infrared rays. True, the sensitive elements located in the skin react directly not to radiation, but to the heating caused by it. It may be that infrared rays do not produce any other effect on the body, but perhaps this is not the case. The final answer will be obtained after solving the riddle of smell.

How does a person, and even more so animals and insects, smell the presence of certain substances at a considerable distance? A simple answer suggests itself: penetrating into the organs of smell, the molecules of a substance cause their own specific irritation of these organs, which we perceive as a certain smell.

But how can one explain this fact: bees flock to honey even when it is hermetically sealed in glass jar. Or another fact: some insects smell at such a low concentration of a substance that on average there is less than one molecule per individual.

In this regard, a hypothesis has been put forward and is being developed, according to which the sense of smell is caused by electromagnetic waves, more than 10 times the wavelength of visible light. These waves are emitted by low-frequency vibrations of molecules and affect the organs of smell. It is curious that this theory brings our eye and nose together in an unexpected way. Both are different types of receivers and analyzers of electromagnetic waves. Whether all this is really so is still quite difficult to say.

5-10. Significant "cloud"

The reader, who throughout this long chapter has probably grown weary of being surprised at the endless variety of manifestations of electromagnetism, penetrating even such a delicate area as perfumery, might conclude that there is no more prosperous theory in the world than this. True, there was some hitch when talking about the structure of the atom. The rest of the electrodynamics seems flawless and invulnerable.

Such a feeling of great well-being arose among physicists at the end of the last century, when the structure of the atom was not yet known. This feeling was so complete that the famous English physicist Thomson, at the turn of the century, seemed to have reason to speak of a cloudless scientific horizon, on which his gaze saw only two "small clouds." It was about Michelson's experiments on measuring the speed of light and the problem of thermal radiation. The results of Michelson's experiments formed the basis of the theory of relativity. Let's talk about thermal radiation in detail.

Physicists were not surprised that all heated bodies radiate electromagnetic waves. It was only necessary to learn how to describe this phenomenon quantitatively, relying on a coherent system of Maxwellian equations and Newton's laws of mechanics. By solving this problem, Rayleigh and Jean got a surprising and paradoxical result. From the theory with complete immutability it followed, for example, that even a human body with a temperature of 36.6 ° C would have to dazzlingly sparkle, inevitably losing energy and rapidly cooling almost to absolute zero.

No subtle experiments are needed here to make sure of a clear conflict between theory and reality. And at the same time, we repeat, the calculations of Rayleigh and Jeans did not raise any doubts. They were direct consequences of the most general statements of the theory. No tricks could save the situation.

The fact that the repeatedly tested laws of electromagnetism went on strike as soon as they were tried to be applied to the problem of radiation of short electromagnetic waves so stunned physicists that they began to talk about the "ultraviolet catastrophe" *). This is what Thomson had in mind when he spoke of one of the "clouds". Why only "cloud"? Yes, because it seemed to physicists at that time that the problem of thermal radiation was a small private issue, not significant against the background of general gigantic achievements.

*) "Catastrophe" was called ultraviolet, since the troubles were associated with radiation of very short waves.

However, this "cloud" was destined to grow and, turning into a giant cloud, obscure the entire scientific horizon, shed an unprecedented downpour that washed away the entire foundation of classical physics. But at the same time, he also brought to life a new physical worldview, which we now briefly designate with two words - "quantum theory".

Before talking about that new one, which to a large extent turned our ideas about both electromagnetic forces and forces in general, let us turn our eyes back and try, from the height to which we have risen, to clearly imagine why electromagnetic forces play in nature has such an outstanding role.

Server rental. Site hosting. Domain names:

New C --- redtram messages:

New posts C---thor:

Almost everything that we know about the cosmos (and the microworld) is known to us thanks to electromagnetic radiation, that is, fluctuations in electric and magnetic fields that propagate in vacuum at the speed of light. Actually, light is a special kind of electromagnetic waves perceived by the human eye.

An exact description of electromagnetic waves and their propagation is given by Maxwell's equations. However, this process can be explained qualitatively without any mathematics. Let's take an electron at rest - almost a point negative electric charge. It creates an electrostatic field around itself, which affects other charges. A repulsive force acts on negative charges, and an attractive force acts on positive charges, and all these forces are directed strictly along the radii coming from our electron. With distance, the influence of an electron on other charges weakens, but never drops to zero. In other words, in the entire infinite space around itself, the electron creates a radial force field (this is true only for an electron that is eternally at rest at one point).

Suppose a certain force (we will not specify its nature) suddenly disturbed the rest of the electron and forced it to move a little to the side. Now the field lines should diverge from the new center where the electron has moved. But electric field, surrounding the charge, cannot instantly rearrange itself. At a sufficiently large distance, the lines of force will point to the initial location of the charge for a long time to come. So it will be until the wave of restructuring of the electric field, which propagates at the speed of light, approaches. This is an electromagnetic wave, and its speed is a fundamental property of space in our Universe. Of course, this description is extremely simplified, and some of it is even simply wrong, but it gives a first impression of how electromagnetic waves propagate.

What is wrong with this description is this. The described process is actually not a wave, that is, a propagating periodic oscillatory process. We have distribution, but there are no hesitation. But this shortcoming is very easy to fix. Let's force the same force that brought the electron out of its original position, immediately return it to its place. Then the first rearrangement of the radial electric field will immediately be followed by the second, restoring the original state of affairs. Now let the electron periodically repeat this movement, and then real waves will run along the radial lines of force of the electric field in all directions. This picture is already much better than the first. However, it is also not entirely true - the waves are purely electric, not electromagnetic.

Here is the time to recall the law of electromagnetic induction: a changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. These two fields seem to be linked to each other. As soon as we create a wave-like change in the electric field, a magnetic wave is immediately added to it. It is impossible to separate this pair of waves - this is a single electromagnetic phenomenon.

You can further refine the description, gradually getting rid of inaccuracies and rough approximations. If we bring this matter to the end, we will just get the already mentioned Maxwell equations. But let's stop halfway, because for now only a qualitative understanding of the issue is important for us, and all the main points are already clear from our model. The main one is the independence of the propagation of an electromagnetic wave from its source.

In fact, the waves of electric and magnetic fields, although they arose due to charge oscillations, but far from it propagate completely independently. Whatever happens to the source charge, the signal about it will not catch up with the outgoing electromagnetic wave - after all, it will propagate no faster than light. This allows us to consider electromagnetic waves as independent physical phenomena along with the charges that generate them.

Frequency and wavelength

An electromagnetic wave is characterized by one main parameter - the number of crests that pass by the observer per second (or enter the detector). This value is called the radiation frequency ν. Since for all electromagnetic waves the speed in vacuum ( With) is the same, it is easy to determine the wavelength λ from the frequency:

λ = With/ν.

We simply divide the path traveled by light per second by the number of oscillations in the same time and get the length of one oscillation. The wavelength is a very important parameter, since it determines the boundary scale: at distances noticeably greater than the wavelength, radiation obeys the laws of geometric optics, it can be described as the propagation of rays. At shorter distances, it is absolutely necessary to take into account the wave nature of light, its ability to flow around obstacles, the impossibility of accurately localizing the position of the beam, etc.

From these considerations, in particular, it follows that it is impossible to obtain an image of objects if their size is on the order of or less than the wavelength of the radiation at which the observation is made. This, in particular, puts a limit on the capabilities of microscopes. In visible light, objects smaller than half a micron cannot be seen; accordingly, an increase of more than 1-2 thousand times for an optical microscope is meaningless.

The history of the discovery of electromagnetic waves

The discovery of electromagnetic waves is a remarkable example of the interaction between experiment and theory. It shows how physics has combined seemingly completely dissimilar properties - electricity and magnetism - revealing in them different aspects of the same physical phenomenon - electromagnetic interaction. Today it is one of the four known fundamental physical interactions, which also include the strong and weak nuclear interactions and gravity. The theory of electroweak interaction has already been constructed, which describes electromagnetic and weak nuclear forces from a unified standpoint. There is also the next unifying theory - quantum chromodynamics - which covers the electroweak and strong interactions, but its accuracy is somewhat lower. describe All Fundamental interactions from a unified position have not yet been achieved, although intensive research is being carried out in this direction within the framework of such areas of physics as string theory and quantum gravity.

Electromagnetic waves were theoretically predicted by the great English physicist James Clark Maxwell (probably for the first time in 1862 in his work "On Physical Lines of Force", although a detailed description of the theory appeared in 1867). He diligently and with great respect tried to translate into strict mathematical language Michael Faraday's somewhat naive pictures describing electrical and magnetic phenomena, as well as the results of other scientists. Having ordered all electrical and magnetic phenomena in the same way, Maxwell discovered a number of contradictions and a lack of symmetry. According to Faraday's law, alternating magnetic fields generate electric fields. But it was not known whether alternating electric fields generate magnetic fields. Maxwell managed to get rid of the contradiction and restore the symmetry of the electric and magnetic fields by introducing an additional term into the equations, which described the appearance of a magnetic field when the electric field changed. By that time, thanks to Oersted's experiments, it was already known that direct current creates a constant magnetic field around the conductor. The new term described another source of the magnetic field, but it could be thought of as some kind of imaginary electric current, which Maxwell called bias current to distinguish from ordinary current in conductors and electrolytes - conduction current. As a result, it turned out that alternating magnetic fields generate electric fields, and alternating electric fields generate magnetic ones. And then Maxwell realized that in such a combination, oscillating electric and magnetic fields can break away from the conductors that generate them and move through vacuum with a certain, but very high speed. He calculated this speed, and it turned out to be about three hundred thousand kilometers per second.

Shocked by the result, Maxwell writes to William Thomson (Lord Kelvin, who, in particular, introduced the absolute temperature scale): “The speed of transverse wave oscillations in our hypothetical medium, calculated from the electromagnetic experiments of Kohlrausch and Weber, coincides so exactly with the speed of light, calculated from optical experiments of Fizeau that we can hardly refuse the conclusion that light consists of transverse vibrations of the same medium, which is the cause of electrical and magnetic phenomena". And further in the letter: “I received my equations while living in the provinces and not suspecting the closeness of the speed of propagation of magnetic effects I found to the speed of light, so I think that I have every reason to consider the magnetic and luminous media as one and the same medium. ... "

Maxwell's equations go far beyond the scope of a school physics course, but they are so beautiful and concise that they should be placed in a conspicuous place in the physics classroom, because most of the natural phenomena that are significant to humans can be described with just a few lines of these equations. This is how information is compressed when previously dissimilar facts are combined. Here is one of the types of Maxwell's equations in differential representation. Admire.

I would like to emphasize that a discouraging consequence was obtained from Maxwell's calculations: the oscillations of the electric and magnetic fields are transverse (which he himself emphasized all the time). And transverse vibrations propagate only in solids, but not in liquids and gases. By that time, it was reliably measured that the speed of transverse vibrations in solids (simply the speed of sound) is the higher, the, roughly speaking, the harder the medium (the greater the Young's modulus and the lower the density) and can reach several kilometers per second. The speed of the transverse electromagnetic wave was almost a hundred thousand times higher than the speed of sound in solids. And it should be noted that the stiffness characteristic is included in the equation for the speed of sound in a solid under the root. It turned out that the medium through which electromagnetic waves (and light) pass has monstrous elastic characteristics. An extremely difficult question arose: “How can other bodies move through such a solid medium and do not feel it?” The hypothetical medium was called - ether, attributing to it at the same time strange and, generally speaking, mutually exclusive properties - enormous elasticity and extraordinary lightness.

Maxwell's work caused shock among contemporary scientists. Faraday himself wrote with surprise: "At first I was even frightened when I saw such a mathematical force applied to the question, but then I was surprised to see that the question withstands it so well." Despite the fact that Maxwell's views overturned all the ideas known at that time about the propagation of transverse waves and about waves in general, far-sighted scientists understood that the coincidence of the speed of light and electromagnetic waves is a fundamental result, which says that it is here that the main breakthrough awaits physics.

Unfortunately, Maxwell died early and did not live to see reliable experimental confirmation of his calculations. International scientific opinion changed as a result of the experiments of Heinrich Hertz, who 20 years later (1886–89) demonstrated the generation and reception of electromagnetic waves in a series of experiments. Hertz not only obtained the correct result in the quiet of the laboratory, but passionately and uncompromisingly defended Maxwell's views. Moreover, he did not limit himself to experimental proof of the existence of electromagnetic waves, but also investigated their basic properties (reflection from mirrors, refraction in prisms, diffraction, interference, etc.), showing the complete identity of electromagnetic waves with light.

It is curious that seven years before Hertz, in 1879, the English physicist David Edward Hughes (Hughes - D. E. Hughes) also demonstrated to other major scientists (among them was also the brilliant physicist and mathematician Georg-Gabriel Stokes) the effect of propagation of electromagnetic waves in the air. As a result of discussions, scientists came to the conclusion that they see the phenomenon of Faraday's electromagnetic induction. Hughes was upset, did not believe himself, and published the results only in 1899, when the Maxwell-Hertz theory became generally accepted. This example shows that in science persistent dissemination and propaganda of the results obtained is often no less important than the scientific result itself.

Heinrich Hertz summed up the results of his experiments in the following way: "The described experiments, as it seems to me at least, eliminate doubts about the identity of light, thermal radiation and electrodynamic wave motion."

grand unification

In the words of Heinrich Hertz, one feels the solemn, albeit restrained, notes of a man who is involved in yet another great unification. It combines into a single entity not only light and electromagnetic waves, but also thermal (now we would say infrared) radiation, which, after the death of Maxwell, was well studied, and its wave nature was proved.

At the end of the 19th century, X-rays (with a huge public outcry) and gamma rays (absolutely unnoticed by the general public) were discovered. It turned out that they also have an electromagnetic wave nature - they are reflected, refracted, diffracted and interfered, like other types of electromagnetic waves. Only their wavelength is much shorter than light, and they interact with matter in a special way.

Ultraviolet radiation was discovered independently in 1801 by the German scientist Johann Wilhelm Ritter and the English scientist William Hyde Wollaston on the photochemical action of ultraviolet radiation on silver chloride. Vacuum ultraviolet was discovered by the German scientist Viktor Schumann using a vacuum spectrograph with a fluorite prism (1885–1903) built by him and gelatin-free photographic plates. American scientist Theodore Lyman built the first vacuum spectrograph with a concave diffraction grating. He was able to register ultraviolet with a wavelength of up to 25 nm (1924).

Gugliermo Marconi, Nikola Tesla and Alexander Stepanovich Popov (among other scientists) learned to transmit information "without wires" using electromagnetic waves of the long-wave part of the spectrum - the radio range. Marconi shocked the world community by transmitting an electromagnetic signal across the ocean in 1901 (which, not without reason, many scientists did not believe, because radio waves of this wavelength could not go around the Earth), thus accidentally opening a huge natural mirror - the ionosphere, from which Marconi's waves are reflected (Nobel Prize 1909). Ten years later, radios have become common household appliances. Human voice and music flooded the world ether, making the transmission of information almost instantaneous and surprisingly cheap (the physical term "ether" has become a common word and term for radio and television broadcasting: "listen to you, Alexander Genrikhovich, you are on the air").

Thus, it turned out that a huge variety of natural phenomena can be reduced to a single phenomenon - electromagnetic waves. Subsequently, very accurate measurements showed that All types of electromagnetic waves move in vacuum with the same speed close to 300 thousand km/s. Moreover, another amazing result was obtained - the speed of electromagnetic waves in vacuum is constant in all reporting systems and it is impossible to exceed it (according to modern concepts) in any physical process. More accurate measurements gave a value With= 299 792 458 m/s with an accuracy of one meter per second. But then it turned out that the accuracy of measuring the speed of light exceeds the accuracy of the length standard - a meter. And then it was decided to consider the above value of the speed of light as accurate by definition, and define the meter as the path traveled by light in vacuum in 1/299,792,458 of a second. The constancy of the speed of light as a fundamental property of the Universe formed the basis of Albert Einstein's special theory of relativity (1905), which opened a series of scientific revolutions in the 20th century.

The only distinguishing characteristic of all types of electromagnetic waves from the radio range to gamma rays was the wavelength (or frequency). The fact that different parts of the electromagnetic spectrum are called differently (light, x-rays, gamma rays, etc.) reminds us that these radiations were initially considered phenomena of a different nature and it took the efforts of dozens of prominent scientists to combine these phenomena into a single entity.

It also turned out that the electromagnetic wave was the only physical wave known at that time that did not need a medium for propagation. This explained the incomprehensible properties of the ether. There is simply no ether as a medium through which electromagnetic waves propagate. He's not needed. Variable electric and magnetic fields, according to classical concepts, generating each other, rush with great speed through empty space.

Maxwell's equations describe the classical behavior of charges and electromagnetic waves. Over time, the equations were rewritten in a four-dimensional form, consistent with the special theory of relativity. But the most developed theory according to modern concepts, which best of all describes the elementary interaction of photons and electrons at the moment, is quantum electrodynamics. This is by far the most accurate theory of electromagnetic waves. In it, the main parameters of the field are momenta and polarizations of photons. The theory makes it possible to calculate the amplitudes of the probabilities of the processes that will occur when interacting with photons and charged particles. Maxwell's classical electrodynamics is a special case of quantum electrodynamics and is derived from it.

Quantum electrodynamics is in excellent agreement with experiment. For its creation, the 1965 Nobel Prize was awarded to Sinichiro Tomonaga, Julius Schwinger, Richard Feynman. But many scientists consider it semi-empirical: “our confidence in the correctness of the results obtained in this way is based, in the final analysis, on their excellent agreement with experience, and not on the internal consistency and logical harmony of the basic principles of the theory” (Richard Feynman). Since the time of Maxwell, physicists have made significant progress in understanding and describing the electromagnetic interaction, but even now a complete theory of electromagnetic interactions has not been created. And those guys who are sitting at their desks today and are interested in physics have a chance to build a logically coherent theory of electromagnetic radiation.

quantum energy

For all classical mechanical waves (in liquids, gases and solids), the main parameter that determines the energy of the wave is its amplitude (more precisely, the square of the amplitude). In the case of light, the amplitude determines the intensity of the radiation. However, when studying the phenomenon of the photoelectric effect - knocking out electrons from a metal by light - it was found that the energy of the knocked-out electrons is not related to the intensity (amplitude) of the radiation, but depends only on its frequency. Even weak blue light knocks electrons out of metal, and the most powerful yellow spotlight cannot knock out a single electron from the same metal. The intensity determines how many electrons will be knocked out - but only if the frequency exceeds some threshold. It turned out that the energy in an electromagnetic wave is fragmented into portions, called quanta. The energy of an electromagnetic radiation quantum is fixed and equal to

E = hν ,

Where h= 4 10 –15 eV· With= 6 10 –34 J· With- Planck's constant, another fundamental physical quantity that determines the properties of our world. A separate quantum interacts with an individual electron during the photoelectric effect, and if its energy is not enough, it cannot knock the electron out of the metal. A long-standing dispute about the nature of light - whether it is waves or a stream of particles - was resolved in favor of a kind of synthesis. Some phenomena are described by wave equations, while others are described by ideas about photons, quanta of electromagnetic radiation, which were put into circulation by two German physicists - Max Planck and Albert Einstein.

The energy of quanta in physics is usually expressed in electron volts. This is an off-system unit of energy measurement. One electron volt (1 eV) is equal to the energy that an electron acquires when it is accelerated by an electric field of 1 volt. This is a very small value, in C units 1 eV= 1.6 10 -19 J. But on the scale of atoms and molecules, an electron volt is quite a solid value.

The ability of radiation to produce a certain effect on matter directly depends on the energy of quanta. Many processes in matter are characterized by a threshold energy - if individual quanta carry less energy, then, no matter how many of them, they will not be able to provoke an above-threshold process.

Looking ahead a bit, let's look at some examples. The energy of microwave quanta is enough to excite the rotational levels of the ground electronic-vibrational state of some molecules, such as water. An energy fraction of an electron volt is enough to excite the vibrational levels of the ground state in atoms and molecules. This determines, for example, the absorption of infrared radiation in the atmosphere. Visible light quanta have an energy of 2–3 eV- this is enough to break chemical bonds and provoke some chemical reactions, for example, those that occur in photographic film and in the retina of the eye. Ultraviolet quanta can break stronger chemical bonds, as well as ionize atoms by stripping off outer electrons. This makes ultraviolet life-threatening. X-ray radiation can pull out electrons from the inner shells of atoms, and also excite vibrations inside atomic nuclei. Gamma radiation is capable of destroying atomic nuclei, and the most energetic gamma rays even penetrate the structure of elementary particles such as protons and neutrons.

Radiation temperature

Finally, there is another way to characterize electromagnetic radiation - by specifying its temperature. Strictly speaking, this method is suitable only for the so-called black-body or thermal radiation. In physics, an absolutely black body is an object that absorbs all radiation falling on it. However, ideal absorbing properties do not prevent the body from emitting radiation itself. On the contrary, for such an idealized body it is possible to accurately calculate the shape of the radiation spectrum. This is the so-called Planck curve, the shape of which is determined by a single parameter - temperature. The famous hump of this curve shows that a heated body radiates little at both very long and very short wavelengths. The maximum radiation falls on a well-defined wavelength, the value of which is directly proportional to the temperature.

When specifying this temperature, one must keep in mind that this is not a property of the radiation itself, but only the temperature of an idealized absolutely black body, which has a maximum radiation at a given wavelength. If there is reason to believe that the radiation is emitted by a heated body, then by finding the maximum in its spectrum, one can approximately determine the temperature of the source. For example, the surface temperature of the Sun is 6,000 degrees. This just corresponds to the middle of the visible radiation range. This is hardly accidental - most likely, the eye during evolution has adapted to make the most efficient use of sunlight.

Temperature ambiguity

The point of the spectrum, which accounts for the maximum of black-body radiation, depends on which axis we are plotting on. If the wavelength in meters is evenly plotted along the abscissa axis, then the maximum will fall on

λ max = b/T= (2.9 10 -3 m· TO)/T ,

Where b= 2.9 10 -3 m· TO. This is the so-called Wien's displacement law. If we plot the same spectrum, plotting the radiation frequency uniformly on the y-axis, the location of the maximum is calculated by the formula:

ν max = (α k/h) · T= (5.9 10 10 Hz/TO) · T ,

where α = 2.8, k= 1.4 10 –23 J/TO is the Boltzmann constant, h is Planck's constant.

Everything would be fine, but as it turns out λ max and v max correspond to different points of the spectrum. This becomes obvious if we calculate the wavelength corresponding to ν max, then you get:

λ’ max = With/ν max = (ch/α k)/T= (5.1 10 -3 m K) / T .

Thus, the maximum of the spectrum, determined by frequency, in λ’ max/ν max = 1,8 times different in wavelength (and hence in frequency) from the maximum of the same spectrum determined from wavelengths. In other words, the frequency and wavelength of the maximum black-body radiation do not correspond to each other: λ max ≠ With/ν max .

In the visible range, it is customary to indicate the maximum of the thermal radiation spectrum along the wavelength. In the spectrum of the Sun, as already mentioned, it falls in the visible range. However, in terms of frequency, the maximum solar radiation lies in the near infrared range.

And here is the maximum of cosmic microwave radiation with a temperature of 2.7 TO it is customary to indicate by frequency - 160 MHz, which corresponds to a wavelength of 1.9 mm. Meanwhile, in the graph by wavelengths, the maximum of the cosmic microwave background falls on 1.1 mm.

All this shows that temperature must be used with great care to describe electromagnetic radiation. It can be used only in the case of radiation close in spectrum to thermal radiation, or for a very rough (up to an order of magnitude) characteristic of the range. For example, visible radiation corresponds to a temperature of thousands of degrees, X-ray - millions, microwave - about 1 kelvin.

Radiation bands and matter

Although electromagnetic waves of all frequencies propagate in a vacuum in the same way - at the speed of light, their interaction with matter depends very much on the frequency (and also on the wavelength and quantum energy). According to the nature of the interaction with matter, radiation is divided into ranges: gamma radiation, x-rays, ultraviolet, visible light, infrared radiation and radio waves, which together form the electromagnetic spectrum. These ranges themselves, in turn, are divided into subranges, and in science there is no single established tradition of such division. Much here depends on the technical means used to generate and detect radiation. Therefore, in each field of science and technology, subranges are defined in their own way, and often even shift the boundaries of the main ranges.

Visible radiation

Of the entire spectrum, the human eye is able to capture radiation only in a very narrow range of visible light. From one edge to the other, the radiation frequency (as well as the wavelength and photon energy) changes by less than a factor of two. For comparison, the longest radio waves are 10 14 times longer than visible radiation, and the most energetic gamma rays are 10 20 times more energetic. Nevertheless, for many thousands of years, most of the information about the world around us was drawn from the visible radiation range, the boundaries of which are determined by the properties of the light-sensitive cells of the human retina.

Different wavelengths of visible light are perceived by humans as different colors - from red to purple. The traditional division of the visible spectrum into seven colors of the rainbow is a cultural convention. There are no clear physical boundaries between colors. The English, for example, usually divide the rainbow into six colors. Other options are also known. Only three different types of receptors are responsible for the perception of the whole variety of colors and shades of visible light, which are sensitive to red, green and blue color. This allows you to reproduce almost any color by mixing these three primary colors on the screen.

To receive visible light from distant cosmic sources, concave mirrors are used, which collect radiation from a large area almost to one point. The larger the mirrors, the more powerful the telescope. Mirrors must be made with extremely high accuracy - deviations from the ideal surface shape should not exceed a tenth of a wavelength - 40 nanometers, that is, 0.04 microns. And such accuracy should be maintained at any rotation of the mirror. This determines the high cost of large telescopes. The diameter of the mirrors of the largest optical instruments - the Keck telescopes in Hawaii - is 10 meters.

Although the atmosphere is transparent to visible light (marked with blue arrows on the poster), it still creates serious interference with observations. Even if you forget about the clouds, the atmosphere slightly bends the rays of light, which reduces the clarity of the image. In addition, the air itself scatters the incident light. During the day, this blue glow, caused by the scattered light of the Sun, does not allow astronomical observations, and at night, the scattered light of the stars (and in recent decades, the artificial illumination of the sky by outdoor lighting from cities, cars, etc.) limits the visibility of the palest objects. To cope with these difficulties allows the removal of telescopes into space. The Hubble telescope, by earthly standards, has a very modest size - a diameter of 2.24 meters, but thanks to its extra-atmospheric placement, it has made it possible to make many first-class astronomical discoveries.

Ultraviolet radiation

On the short-wavelength side of visible light is the ultraviolet range, which is divided into near and vacuum. Like visible light, near ultraviolet passes through the atmosphere. A person does not perceive it with the senses, but near ultraviolet radiation causes a tan on the skin. This is a protective reaction of the skin to certain chemical disorders under the influence of ultraviolet radiation. The shorter the wavelength, the more disruption ultraviolet radiation can cause in biological molecules. If all ultraviolet light passed through the atmosphere, life on the Earth's surface would be impossible. However, above a certain frequency, the atmosphere ceases to transmit ultraviolet radiation, since the energy of its quanta becomes sufficient to destroy (dissociate) air molecules. One of the first ultraviolet shocks is ozone, followed by oxygen. Together, atmospheric gases protect the Earth's surface from the harsh ultraviolet radiation of the Sun, which is called vacuum radiation, since it can only propagate in emptiness (vacuum). Upper limit of vacuum ultraviolet - 200 nm. From this wavelength, molecular oxygen (O 2) begins to absorb ultraviolet light.

Telescopes for near ultraviolet radiation are built according to the same principles as for the visible range. They also use mirrors coated with a thin reflective metal layer, but they must be made with even greater precision. Near ultraviolet can be observed from the Earth, vacuum - only from space.

x-ray radiation

There is no formal boundary between hard ultraviolet and x-ray radiation. There are two main approaches to its definition: on the one hand, it is customary to refer to X-rays as radiation capable of causing excitation of atomic nuclei - just as visible and infrared radiation excites the electron shells of atoms and molecules. In this case, even hard vacuum ultraviolet can in some cases be referred to as x-rays. In another approach, radiation with a wavelength less than the characteristic size of atoms (0.1 nm). Then it turns out that most of the soft X-ray range should be considered superhard ultraviolet.

Soft X-rays can still be reflected from polished metal, but only with grazing incidence - at an angle of less than 1 degree. Harder radiation has to be concentrated in other ways. To set the direction, narrow tubes are used that cut off quanta coming from the side, and the receiver is a scintillator in which X-ray quanta ionize atoms, and those, recombining with electrons, emit visible or ultraviolet radiation, which is recorded using photoelectron multipliers. In fact, in hard X-ray telescopes, individual radiation quanta are counted and only then an image is formed using a computer.

From x-ray to gamma

The boundary at which the X-ray range is replaced by gamma radiation is also conditional. Usually it is associated with the energy of quanta that are emitted during nuclear reactions (or vice versa, they can cause them). Another approach is related to the fact that thermal radiation is not usually attributed to the gamma range, no matter how high its energy is. Relatively stable macroscopic objects are observed in the Universe, heated to tens of millions of degrees - these are the central sections of accretion disks around neutron stars and black holes. But objects with a temperature of billions of degrees - for example, the core of massive red giants - are almost always covered with an opaque shell. However, often even the radiation in their depths is called not soft gamma radiation, but superhard X-rays. Stable formations with temperatures above tens of billions of degrees are unknown in the modern Universe. This gives reason to believe that gamma radiation is always generated in a non-thermal way. The main mechanism is radiation from the collision of charged particles accelerated to near-light speeds by powerful electromagnetic fields, for example, in neutron stars.

Gamma radiation

The division of gamma radiation into subbands is even more conditional. Ultra-high energies include gamma quanta, the generation of which is beyond the capabilities modern technologies. All sources of such radiation are associated exclusively with space. But since technology tends to evolve, this definition cannot be called clear.

The atmosphere also protects us from gamma radiation. In the soft and hard subranges, it completely absorbs it. Ultrahigh energy quanta, colliding with the nuclei of atoms in the atmosphere, generate cascades of particles, the energy of which gradually decreases and dissipates. However, the first echelons of particles in them move faster than the speed of light. in the air. Under such conditions, charged particles generate the so-called bremsstrahlung (Cherenkov) radiation, somewhat similar to a sonic shock wave from a supersonic aircraft. Ultraviolet and visible bremsstrahlung quanta reach the Earth's surface, where they are captured by special telescopes. It can be said that the atmosphere itself becomes part of the telescope, and this makes it possible to observe superhigh-energy gamma radiation from the Earth. This is marked on the poster with red arrows.

Even more energetic quanta - ultra-high energies - generate such powerful cascades of particles that they pierce the atmosphere through and reach the Earth's surface. They are called extensive air showers (EAS) and are recorded by scintillation sensors. EAS particles, along with the natural radioactivity of terrestrial rocks, can damage biological molecules, in particular DNA, and cause mutations in living organisms. In doing so, they contribute to the evolution of life on Earth. But if their intensity were noticeably higher, this could become a serious obstacle to life. Fortunately, the higher the energy of gamma rays, the rarer they are. The most energetic quanta with an energy of about 10 20 eV come about once every hundred years per square kilometer of the earth's surface. The origin of such energetic gamma quanta is still not entirely clear. The quanta cannot have much higher energy, since above a certain threshold they begin to interact with the relic microwave radiation, leading to the creation of charged particles. In other words, the Universe is opaque to radiation noticeably more energetic than 10 21 –10 24 eV.

Infrared radiation

Going from visible light to the long-wavelength side of the spectrum, we fall into the infrared range. Near infrared radiation is physically no different from visible light, except that it is not perceived by the retina. It can be registered by the same instruments, in particular, telescopes, as visible light. A person also feels infrared radiation with the skin - as heat. It is thanks to infrared radiation that we are warm to sit by the fire. Most of the combustion energy is carried away upward by the upward flow of air, on which we boil water in a pot, and infrared (and visible) radiation is emitted to the sides by gas molecules, combustion products and hot coal particles.

As the wavelength increases, the atmosphere becomes less transparent to infrared radiation. This is due to the so-called vibrational-rotational absorption bands of atmospheric gas molecules. Being quantum objects, molecules cannot rotate or oscillate arbitrarily, like weights on a spring. Each molecule has its own set of energies (and, accordingly, radiation frequencies), which they can store in the form of vibrational and rotational movements. However, even for not the most complex air molecules, the set of these frequencies is so extensive that, in fact, the atmosphere absorbs all radiation in some parts of the infrared spectrum - these are the so-called infrared absorption bands. They are interspersed with small areas in which cosmic infrared radiation reaches the Earth's surface - these are the so-called transparency windows, of which there are about a dozen. Their existence is represented on the poster by scattered blue arrows in infrared. It is interesting to note that the absorption of IR radiation almost completely occurs in the lower layers of the atmosphere due to the increase in air density near the Earth's surface. This allows observations in almost the entire infrared range from balloons and high-altitude aircraft that rise into the stratosphere.

The division of infrared radiation into subranges is also very conditional. The boundary between the near and middle infrared radiation is drawn approximately in the region of the absolute temperature of 300 K, which is typical for objects on the earth's surface. Therefore, all of them, including devices, are powerful sources of infrared radiation. In order to isolate the radiation of a cosmic source under such conditions, the equipment has to be cooled to temperatures close to absolute zero and taken out of the atmosphere, which itself shines intensively in the mid-IR range - it is due to this radiation that the Earth dissipates into space the energy constantly coming from Sun. The main type of radiation receiver in this range is a bolometer, that is, simply put, a small black body that absorbs radiation, connected to an ultra-precise thermometer.

The far infrared range is one of the most difficult, both for generating and for detecting radiation. Recently, thanks to the development of special materials and ultra-fast electronics, they have learned to work with it quite efficiently. In engineering, it is often called terahertz radiation. Currently, the development of non-contact scanners for determining chemical composition objects based on terahertz radiation generators. They will be able to detect plastic explosives and drugs at airport checkpoints.

In astronomy, this range is often referred to as submillimeter radiation. It is interesting because in it (as well as in the microwave range adjacent to it) the relic radiation of the Universe is observed. Submillimeter radiation does not reach sea level, but it is absorbed mainly in the lowest layers of the atmosphere. Therefore, in the mountains of Chile and Mexico at an altitude of about 5 thousand meters above sea level, large submillimeter telescopes are now being built - in Mexico, 50-meter, and in Chile, an array of 64 telescopes with a diameter of 12 meters.

Microwaves and radio waves

The infrared range is adjacent to radio emission, which covers the entire long-wavelength edge of the electromagnetic spectrum. The energy of quanta in the radio range is very small. It is usually not enough for significant changes in the structure of atoms and molecules, but it is enough to interact with the rotational levels of molecules, for example, water. The energy of radio waves is also sufficient to act on free electrons, for example, in conductors. Fluctuations in the electromagnetic field of the radio wave cause synchronous oscillations of the electrons in the antenna, that is, an alternating electric current.

At high intensity of microwave radiation, this current can cause significant heating of the substance. This property is used to heat foods containing water in microwave ovens. Microwave radiation is also referred to as microwave radiation. It is the shortest wavelength subband of radio emission with a wavelength of 1 mm up to 30 cm. Microwave radiation penetrates into the thickness of the products to a depth of several centimeters, which ensures heating throughout the volume, and not just from the surface, as is the case with infrared radiation on the grill. All cell phone and local radio communication systems, such as the Bluetooth and WiFi protocols used by wireless electronic devices, also operate in the microwave range.

The longer the radio wave, the less energy it carries and the more difficult it is to register. For reception, an antenna in which electrical oscillations occur under the action of a radio wave is connected to an electrical circuit. When hit in resonance with its own frequency, the oscillations are amplified and they can be registered. To catch radio waves coming from space, parabolic-shaped antenna mirrors are used, which collect radio emission over their entire area and concentrate it on a small antenna. This increases the sensitivity of the instrument.

Most of the microwave radiation (beginning with a wavelength of 3–5 mm) passes through the atmosphere. The same can be said about ultrashort waves (VHF), on which local television and radio stations (including FM stations) broadcast and space radio communications are conducted. The radiation of their transmitters is recorded only within the line of sight of the antennas. The atmospheric transparency window in the radio range (blue arrows on the poster) ends at about a wavelength of 10–30 meters.

Longer radio waves are reflected from the Earth's ionosphere. This does not allow observation of cosmic radio sources at longer wavelengths, but it does provide the possibility of global shortwave radio communication. Radio waves in the range from 10 to 100 meters can go around the entire Earth, reflecting many times from the ionosphere and the Earth's surface. True, their distribution depends on the state of the ionosphere, which is strongly influenced by solar activity. Therefore, shortwave communication is not of high quality and reliability.

Medium and long waves are also reflected from the ionosphere, but attenuate more strongly with distance. In order for the signal to be caught at a distance of more than a thousand kilometers, very powerful transmitters are required. Ultra-long radio waves, with a length of hundreds and thousands of kilometers, go around the Earth no longer due to the ionosphere, but due to wave effects, which also allow them to penetrate to some depth under the surface of the ocean. This feature is used for emergency communications with submerged combat submarines. Other radio waves do not pass through sea water, which, due to the salts dissolved in it, is a good conductor and absorbs or reflects radio radiation.

No theoretical limit for the length of radio waves is known. In practice, it was experimentally possible to create and register a radio wave with a wavelength of 38 thousand km. km(frequency 8 Hz).

What is on the poster

In the center of the whole composition is a man. The vertical axis corresponds to the visible range - the only one directly perceived by a person. On the left is the short-wave part of the spectrum (ultraviolet, x-ray, gamma), on the right - long-wave (infrared radiation and radio waves with a specially allocated sub-range of microwave radiation). Spectrum bandwidth divided into ranges and subranges, on the boundaries of which the values ​​of the wavelength are indicated (and on the "paper" version of the poster - also the frequencies, quantum energies and temperatures of a possible source of radiation, if it is thermal). The scale is not observed when marking, but the relative sizes of the ranges are taken into account.

To emphasize the conventionality of the boundaries between the ranges, the color transitions between them are made smooth. The radio and gamma bands located at the edges of the spectrum are shown expanding and fading away, which indicates their huge size compared to the rest of the ranges and the absence of external boundaries.

The poster has a multi-level vertical structure. The levels are signed on the poster on the left; when moving from the bottom up, they correspond to a shift from the engineering-pragmatic aspect of the application of various radiations to the disinterested-cognitive one.

On the left and right on the spectrum strip, the basic concepts and relationships necessary to understand the notation on the scale are explained.

Electromagnetic radiation is the main, but not the only source of our knowledge about space. To draw attention to this, information about is given in a separate column on the right.

Radiation receivers

The base level is the conditional surface of the Earth, on which children stand and there is a row astronomical instruments that can work on Earth. Above is the level of spacecraft used for transatmospheric observations. In some ranges, space observations are the only way to obtain information about the Universe.

Ground-based astronomical instruments(from left to right):

• Many gamma, x-ray, etc. telescopes use photomultiplier tubes (PMTs) combined into matrices
• the sky can be carried out without special instruments or with the simplest (for example, through binoculars)
• 24-meter Magellan optical telescope (under construction; visible, and infrared range)
• ALMA radio telescope system (64 telescopes, under construction)

space instruments:

• Gamma Ray Observatory INTEGRAL (INTERnational Gamma-Ray Astrophysics Laboratory; also used in x-ray)
• (visible range, ultraviolet, IR)
• Space gravitational telescope LISA (project, )

Underground tools:

• (outside the electromagnetic spectrum)

Transparency windows

Above the instruments there is a spectrum band, which has already been mentioned above. Directly below it, arrows mark the so-called transparency windows- the ranges in which observations can be made from the surface of the Earth. Blue arrows - windows in which the radiation directly reaches the earth's surface; red arrows in the gamma range indicate the possibility of observing from the Earth secondary effects generated by radiation in the atmosphere.

sky surveys

The band of the spectrum graphically separates the sphere of active human activity from the sphere of celestial bodies, which are available only for passive observation. Immediately above the spectrum is the row sky views. These images show what the celestial sphere looks like at different ranges.

The colors in all images (except for the visible range) are conditional. All reviews are made in the projection traditionally used to represent the world map. The plane of our Galaxy, the Milky Way, has been chosen everywhere as the equator. In almost all ranges, it is the most visible object in the sky. For each review, its astronomical designation is indicated, by which additional information can be found on the Internet.

Poster Sky Surveys

• MeV(CGRO-COMPTEL)
• Near infrared sky 1–4 micron(COBE/DIRBE)
• Mid infrared sky 25 micron(COBE/DIRBE)
• Radio sky on wave 21 cm, 1420 MHz(Dickey & Lockman)

Sources

Finally, above the reviews are examples radiation sources- space objects that can be observed in the corresponding ranges of the spectrum. In most cases, they are represented by real images obtained during astronomical observations. A few exceptions are noted in the captions.

Due to the special importance of the visible range, despite its narrowness, an extended space is specially allocated for it. Objects there from left to right are placed in order of increasing mass.

• Supernova remnant in superhigh-energy gamma rays
• (rice. artist)
• An accretion disk in a close binary system ( rice. artist)
• Solar prominences in X-ray
• Aurora on Jupiter in ultraviolet
• in the visible range
• In the infrared, the Hubble telescope can see more galaxies than stars
• The Sombrero Galaxy in infrared
• Nebulae and dust clouds near the center of the Galaxy in infrared
• Crab nebula in the radio band

Earth application

The last level of the poster includes examples terrestrial application various types of radiation, showing how electromagnetic radiation of various ranges is used in technology. This direction actually lies outside the main theme of the poster. man, radiation and the universe, but the appearance of these examples marks another important line man, radiation and technology.

Diagrams and graphs

In several places on the poster there are leaflets with simple hand-drawn sketches. Their purpose is to illustrate mechanisms that are not clear from photographs or other realistic representations.

Basic ratios and units of measurement

λ = c/ν
E = h ·ν

λ ( lambda) - wavelength

Unit: 1 m(meter);

1 micron = 10 –6 m- micron, micrometer;
1 nm = 10 –9 m- nanometer

ν ( nude) - frequency

Unit: 1 Hz- one oscillation per second;

1 kHz = 1000 Hz- kilohertz;
1 MHz = 10 6 Hz = 1 000 000 Hz- megahertz;
1 GHz = 10 9 Hz= 1,000,000,000 - gigahertz

E- energy

Unit: 1 eV= 1.6 10 -19 J- electron volt, the energy of an electron that has passed through a potential difference of 1 volt;

1 keV = 1000 eV- kiloelectronvolt;
1 MeV = 10 6 eV = 1 000 000 eV- megaelectronvolt;
1 GeV = 10 9 eV = 1 000 000 000 eV- gigaelectronvolt

T- blackbody temperature

Unit: 1 TO- kelvin, degrees Kelvin.
Counted from absolute zero; ice melting point - 273 TO= 0°С; boiling point of water - 373 TO= 100°С

With= 3 10 8 m/s = 300 000 km/s- speed of light

h= 4 10 –15 eV· With- Planck's constant

Gamma radiation

Sources

An accretion disk around a supermassive black hole ( rice. artist)

During the evolution of large galaxies, supermassive black holes are formed in their centers, with a mass from several million to billions of solar masses. They grow due to the accretion (fall) of interstellar matter and even entire stars onto a black hole. With intense accretion, a rapidly rotating disk is formed around the black hole (due to the conservation of the moment of rotation of the matter falling into the hole). Due to the viscous friction of layers rotating at different speeds, it heats up all the time and begins to radiate in the X-ray range. Part of the matter during accretion can be ejected in the form of jets (jets) along the axis of the rotating disk. This mechanism ensures the activity of the nuclei of galaxies and quasars. There is also a black hole at the core of our Galaxy (the Milky Way). At present, its activity is minimal, but according to some indications, about 300 years ago it was much higher.

Receivers

Located in Namibia, it consists of 4 parabolic dishes with a diameter of 12 meters, placed on a platform measuring 250 meters. Each of them has 382 round mirrors with a diameter of 60 cm, which concentrate the bremsstrahlung generated by the motion of energetic particles in the atmosphere (see the diagram of the telescope). The telescope began operating in 2002. It can equally be used to detect energetic gamma quanta and charged particles - cosmic rays. One of his main results was a direct confirmation of the long-standing assumption that supernova remnants are sources of cosmic rays.

When an energetic gamma ray enters the atmosphere, it collides with the nucleus of one of the atoms and destroys it. In this case, several fragments of the atomic nucleus and gamma quanta of lower energy are generated, which, according to the law of conservation of momentum, move almost in the same direction as the original gamma ray. These debris and quanta soon collide with other nuclei, forming an avalanche of particles in the atmosphere. Most of these particles travel faster than the speed of light in air. As a result, the particles emit bremsstrahlung, which reaches the Earth's surface and can be detected by optical and ultraviolet telescopes. In fact, the earth's atmosphere itself serves as an element of the gamma-ray telescope. For ultrahigh-energy gamma rays, the divergence of the beam reaching the Earth's surface is about 1 degree. This determines the resolution of the telescope. At an even higher energy of gamma rays, an avalanche of particles itself reaches the surface - an extensive air shower (EAS). They are recorded by scintillation sensors. An observatory named after Pierre Auger (in honor of the discoverer of the EAS) is currently being built in Argentina to observe gamma radiation and ultra-high-energy cosmic rays. It will include several thousand tanks of distilled water. PMTs installed in them will monitor flashes occurring in water under the influence of energetic EAS particles.

Electronic device for measuring weak fluxes of visible and ultraviolet radiation. PMT is a vacuum tube with a photocathode and a set of electrodes, to which a sequentially increasing voltage is applied with a total drop of up to several kilovolts. Radiation quanta fall on the photocathode and knock out electrons from it, which move towards the first electrode, forming a weak photoelectric current. However, along the way, the electrons are accelerated by the applied voltage and knock out a much larger number of electrons from the electrode. This is repeated several times - according to the number of electrodes. As a result, the electron flow that came from the last electrode to the anode increases by several orders of magnitude compared to the initial photoelectric current. This allows you to register very weak light fluxes, up to individual quanta. An important feature of the PMT is the response speed. This allows them to be used to detect transient phenomena, such as flashes that occur in a scintillator when an energetic charged particle or quantum is absorbed.

sky surveys

Sky in gamma rays with energy 100 MeV(CGRO)

The sky in gamma rays with an energy of 1.8 MeV(CGRO-COMPTEL)

Earth application

x-ray

Sources

The Crab Nebula is the remnant of a supernova that occurred in 1054. The nebula itself is a shell of a star scattered in space, and its core compressed and formed a superdense rotating neutron star with a diameter of about 20 km.The rotation of this neutron star is tracked by strictly periodic oscillations of its radiation in the radio range. But the pulsar also emits in the visible and X-ray ranges. In x-rays, the Chandra telescope was able to image an accretion disk around a pulsar and small jets perpendicular to its plane (cf. an accretion disk around a supermassive black hole).

An accretion disk in a close binary system ( rice. artist)

The visible surface of the Sun is heated to about 6 thousand degrees, which corresponds to the visible range of radiation. However, the corona surrounding the Sun is heated to a temperature of more than a million degrees and therefore glows in the X-ray range of the spectrum. This image was taken during the maximum solar activity, which varies with a period of 11 years. The very surface of the Sun in X-rays practically does not radiate and therefore looks black. During solar minimum, the X-ray emission from the Sun is significantly reduced. The image was taken by the Japanese Yohkoh (“Sunbeam”) satellite, also known as Solar-A, which operated from 1991 to 2001.

Receivers

One of the four "Great observatories" NASA, named after the American astrophysicist of Indian origin Subramanyan Chandrasekhar (1910–95), Nobel Prize winner (1983), specialist in the theory of the structure and evolution of stars. The main instrument of the observatory is an oblique incidence X-ray telescope with a diameter of 1 .2 m, which contains four nested oblique parabolic mirrors (see diagram) that turn into hyperbolic ones. The observatory was launched into orbit in 1999 and operates in the soft X-ray range (100 eV-10 keV). Among Chandra's many discoveries is the first image of an accretion disk around a pulsar in the Crab Nebula.

sky surveys

Microwave sky 1.9 mm(WMAP)

The cosmic microwave background, also called the cosmic microwave background, is the cooled glow of the hot Universe. It was first discovered by A. Penzias and R. Wilson in 1965 (Nobel Prize in 1978). The first measurements showed that the radiation is completely uniform throughout the sky. This result was obtained by the Soviet satellite "Relikt-1" and confirmed by the American satellite COBE (see Sky in infrared). COBE has also determined that the CMB spectrum is very close to blackbody. The 2006 Nobel Prize was awarded for this result. Variations in the brightness of the cosmic microwave background radiation across the sky do not exceed one hundredth of a percent, but their presence indicates barely noticeable inhomogeneities in the distribution of matter that existed on early stage evolution of the Universe and served as the embryos of galaxies and their clusters. However, the accuracy of the COBE and Relikt data was not enough to test cosmological models, and therefore in 2001 a new more accurate WMAP (Wilkinson Microwave Anisotropy Probe) apparatus was launched, which by 2003 had built a detailed a map of the distribution of the intensity of the background radiation over the celestial sphere. On the basis of these data, the cosmological models and ideas about the evolution of galaxies are now being refined.

Relic radiation arose when the age of the Universe was about 400 thousand years and, due to expansion and cooling, it became transparent to its own thermal radiation. Initially, the radiation had a Planck (black-body) spectrum with a temperature of about 3000 K and accounted for the near infrared and visible ranges of the spectrum. As the Universe expanded, the cosmic microwave background experienced a redshift, which led to a decrease in its temperature. At present, the temperature of the background radiation is 2.7 TO and it falls on the microwave and far infrared (submillimeter) ranges of the spectrum. The graph shows an approximate view of the Planck spectrum for this temperature. The CMB spectrum was measured for the first time by the COBE satellite (see Infrared Sky), for which the Nobel Prize was awarded in 2006.

Radio sky on wave 21 cm, 1420 MHz(Dickey & Lockman)

The famous spectral line with a wavelength of 21.1 cm is another way to observe neutral atomic hydrogen in space. The line arises due to the so-called hyperfine splitting of the ground energy level of the hydrogen atom. The energy of an unexcited hydrogen atom depends on the mutual orientation of the proton and electron spins. If they are parallel, the energy is slightly higher. Such atoms can spontaneously transition to a state with antiparallel spins, emitting a radio emission quantum that carries away a tiny excess of energy. With a single atom, this happens on average once every 11 million years. But the huge distribution of hydrogen in the universe makes it possible to observe gas clouds at this frequency.

Radio sky on a wave of 73.5 cm, 408 MHz(Bonn)

Earth application

Main advantage microwave oven- heating over time of products throughout the volume, and not just from the surface. Microwave radiation, having a longer wavelength, penetrates deeper than infrared under the surface of products. Inside the food, electromagnetic vibrations excite the rotational levels of water molecules, the movement of which basically causes the food to heat up. Thus, microwave (MW) drying of products, defrosting, cooking and heating takes place. Also, alternating electric currents excite high-frequency currents. These currents can occur in substances where mobile charged particles are present. But sharp and thin metal objects should not be placed in a microwave oven (this is especially true for dishes with sprayed metal decorations for silver and gold). Even a thin ring of gilding along the edge of the plate can cause a powerful electrical discharge that will damage the device that creates an electromagnetic wave in the furnace (magnetron, klystron).

The principle of operation of cellular telephony is based on the use of a radio channel (in the microwave range) for communication between the subscriber and one of the base stations. Information is transmitted between base stations, as a rule, via digital cable networks. The range of a base station - the size of a cell - is from several tens to several thousand meters. It depends on the landscape and on the signal strength, which is selected so that there are not too many active subscribers in one cell. In the GSM standard, one base station can provide no more than 8 telephone conversations at the same time. At mass events and during natural disasters, the number of callers increases dramatically, which overloads the base stations and leads to interruptions in cellular communications. In such cases, cellular operators have mobile base stations that can be quickly delivered to an area with a large crowd of people. There is a lot of controversy about the possible harm of microwave radiation from cell phones. During a conversation, the transmitter is in close proximity to the person's head. Repeatedly conducted studies have not yet been able to reliably register the negative effects of radio emission from cell phones on health. Although it is impossible to completely exclude the effect of weak microwave radiation on body tissues, there are no grounds for serious concern.

The television image is transmitted on meter and decimeter waves. Each frame is divided into lines, along which the brightness changes in a certain way. The transmitter of the television station constantly broadcasts a radio signal of a strictly fixed frequency, it is called the carrier frequency. The receiving circuit of the TV is adjusted to it - a resonance occurs in it at the desired frequency, which makes it possible to capture weak electromagnetic oscillations. Information about the image is transmitted by the amplitude of oscillations: large amplitude - high brightness, low amplitude - a dark area of ​​the image. This principle is called amplitude modulation. Sound is transmitted by radio stations in a similar way (except for FM stations). With the transition to digital television, the image coding rules change, but the very principle of the carrier frequency and its modulation is preserved.

Parabolic antenna for receiving a signal from a geostationary satellite in the microwave and VHF bands. The principle of operation is the same as radio telescope, but the dish does not need to be made movable. At the time of installation, it is sent to the satellite, which always remains in the same place relative to earth structures. This is achieved by placing the satellite into a geostationary orbit with a height of about 36 thousand meters. km over the earth's equator. The period of revolution along this orbit is exactly equal to the period of rotation of the Earth around its axis relative to the stars - 23 hours 56 minutes 4 seconds. The size of the dish depends on the power of the satellite transmitter and its radiation pattern. Each satellite has a main service area where its signals are received by a dish with a diameter of 50–100 cm, and the peripheral zone, where the signal weakens rapidly and may require an antenna of up to 2–3 m.

Beyond the electromagnetic spectrum

A person receives most of the information through vision, that is, by capturing electromagnetic radiation in a narrow range of visible light. The same can be said about astronomers, only the range of orders available to them is 30 orders of magnitude wider. But electromagnetic radiation is not the only channel for obtaining information.

A person feels the warmth of close heated objects, and astronomers register neutrinos - subtle particles that are born in countless quantities in the depths of stars, including the Sun, and freely go outside.

A person perceives odors carried by volatile substances. An analogue in astronomy is cosmic rays - energetic charged particles, mainly protons, which accelerate to enormous speeds in various cosmic cataclysms, and then reach the Earth.

A person has a sense of touch, and astronomers can feel the cosmic substance - meteorites that have fallen to Earth, the soil of neighboring celestial bodies, just particles of dust and gas collected in space.

And very soon astronomy should acquire an analogue of hearing - the ability to register gravitational waves, fluctuations in space itself, generated by sharp movements of huge masses, for example, neutron stars and black holes.

Space Gravity Telescope LISA ( project)

poster again

), which describes the electromagnetic field, theoretically showed that an electromagnetic field in a vacuum can exist even in the absence of sources - charges and currents. A field without sources has the form of waves propagating at a finite speed, which in vacuum is equal to the speed of light: With= 299792458±1.2 m/s. The coincidence of the speed of propagation of electromagnetic waves in vacuum with the speed of light measured earlier allowed Maxwell to conclude that light is electromagnetic waves. This conclusion later formed the basis of the electromagnetic theory of light.

In 1888, the theory of electromagnetic waves received experimental confirmation in the experiments of G. Hertz. Using a high voltage source and vibrators (see Hertz vibrator), Hertz was able to perform subtle experiments to determine the speed of propagation of an electromagnetic wave and its length. It was experimentally confirmed that the speed of propagation of an electromagnetic wave is equal to the speed of light, which proved the electromagnetic nature of light.

Spectrum of electromagnetic waves.

Electromagnetic waves are classified according to the lambda wavelength or the associated f wave frequency. We also note that these parameters characterize not only the wave, but also the quantum properties of the electromagnetic field. Accordingly, in the first case, the electromagnetic wave is described by classical laws, studied in this volume, and in the second case, by quantum laws, studied in Volume 5 of this manual.

Consider the concept of the spectrum of electromagnetic waves. The spectrum of electromagnetic waves called the frequency band of electromagnetic waves that exist in nature.

The spectrum of electromagnetic radiation in order of increasing frequency is:

1) Radio waves;

2) Infrared radiation;

3) Light emission;

4) X-ray radiation;

5) Gamma radiation.

Different sections of the electromagnetic spectrum differ in the way they emit and receive waves belonging to one or another section of the spectrum. For this reason, there are no sharp boundaries between different parts of the electromagnetic spectrum.

Radio waves are studied by classical electrodynamics. Infrared light and ultraviolet radiation are studied both by classical optics and quantum physics. X-ray and gamma radiation is studied in quantum and nuclear physics.

Let us consider the spectrum of electromagnetic waves in more detail.

Radio waves.

radio waves are electromagnetic waves whose lengths exceed 0.1 mm (frequency less than 3 10 12 Hz = 3000 GHz).

Radio waves are divided into:

1. Ultra-long waves with a wavelength greater than 10 km (frequency less than 3 10 4 Hz = 30 kHz);

2. Long waves in the length range from 10 km to 1 km (frequency in the range 3 10 4 Hz - 3 10 5 Hz = 300 kHz);

3. Medium waves in the length range from 1 km to 100 m (frequency in the range 3 10 5 Hz -310 6 Hz = 3 MHz);

4. Short waves in the wavelength range from 100m to 10m (frequency in the range of 310 6 Hz-310 7 Hz=30 MHz);

5. Ultrashort waves with a wavelength less than 10m (frequency more than 310 7 Hz = 30 MHz).

Ultrashort waves, in turn, are divided into:

a) meter waves;

b) centimeter waves;

c) millimeter waves;

d) submillimeter or micrometer.

Waves with a wavelength less than 1m (frequency less than 300MHz) are called microwaves or microwaves.

Due to the large values ​​of the wavelengths of the radio range compared to the size of atoms, the propagation of radio waves can be considered without taking into account the atomistic structure of the medium, i.e. phenomenologically, as is customary in the construction of Maxwell's theory. The quantum properties of radio waves are manifested only for the shortest waves adjacent to the infrared part of the spectrum and during the propagation of the so-called. ultrashort pulses with a duration of the order of 10 -12 sec - 10 -15 sec, comparable with the time of oscillations of electrons inside atoms and molecules.

Infrared and light radiation.

infrared, light, including ultraviolet, the radiations are optical region of the spectrum of electromagnetic waves in the broadest sense of the word. The closeness of the sections of the spectrum of these waves led to the similarity of the methods and instruments used for their study and practical application. Historically, lenses, diffraction gratings, prisms, diaphragms, optically active substances that are part of various optical devices (interferometers, polarizers, modulators, etc.) were used for these purposes.

On the other hand, the radiation of the optical region of the spectrum has general patterns of passage of various media, which can be obtained using geometric optics, which is widely used for calculations and construction of both optical devices and optical signal propagation channels.

The optical spectrum occupies a range of electromagnetic wave lengths in the range from 210 -6 m = 2 μm to 10 -8 m = 10nm (in frequency from 1.510 14 Hz to 310 16 Hz). Upper limit of the optical range determined by the long-wavelength boundary of the infrared range, and lower shortwave ultraviolet limit(Fig.2.14).

Optical frequency bandwidth is approximately 18 octaves 1 , of which the optical range accounts for approximately one octave (); for ultraviolet - 5 octaves (), for infrared radiation - 11 octaves (

In the optical part of the spectrum, phenomena due to the atomistic structure of matter become significant. For this reason, along with the wave properties of optical radiation, quantum properties appear.

X-ray and gamma radiation.

In the field of X-ray and gamma radiation, the quantum properties of radiation come to the fore.

x-ray radiation arises during the deceleration of fast charged particles (electrons, protons, etc.), as well as as a result of processes occurring inside the electron shells of atoms.

Gamma radiation is a consequence of phenomena occurring inside atomic nuclei, as well as as a result of nuclear reactions. The boundary between X-ray and gamma radiation is determined conditionally by the magnitude of the energy quantum 2 corresponding to a given radiation frequency.

X-ray radiation consists of electromagnetic waves with a length of 50 nm to 10 -3 nm, which corresponds to a quantum energy of 20 eV to 1 MeV.

Gamma radiation is electromagnetic waves with a wavelength less than 10 -2 nm, which corresponds to a photon energy greater than 0.1 MeV.

electromagnetic nature of light.

Light represents the visible part of the spectrum of electromagnetic waves, the wavelengths of which occupy the interval from 0.4 µm to 0.76 µm. Each spectral component of optical radiation can be associated with a specific color. Coloring of spectral components of optical radiation determined by their wavelength. The color of the radiation changes as its wavelength decreases as follows: red, orange, yellow, green, cyan, indigo, violet.

The red light corresponding to the longest wavelength defines the red end of the spectrum. Violet light - corresponds to the purple border.

natural light is not colored and represents a superposition of electromagnetic waves from the entire visible spectrum. Natural light comes from the emission of electromagnetic waves by excited atoms. The nature of excitation can be different: thermal, chemical, electromagnetic, etc. As a result of excitation, atoms emit electromagnetic waves in a chaotic manner for about 10 -8 seconds. Since the energy spectrum of excitation of atoms is quite wide, electromagnetic waves are emitted from the entire visible spectrum, the initial phase, direction and polarization of which is random. For this reason, natural light is not polarized. This means that the "density" of the spectral components of electromagnetic waves of natural light having mutually perpendicular polarizations is the same.

Harmonic electromagnetic waves in the light range are called monochromatic. For a monochromatic light wave, one of the main characteristics is the intensity. light wave intensity is the average value of the energy flux density (1.25) carried by the wave:

where is the Poynting vector.

Calculation of the intensity of a light, plane, monochromatic wave with an electric field amplitude in a homogeneous medium with dielectric and magnetic permeability according to the formula (1.35) taking into account (1.30) And (1.32) gives:

where is the refractive index of the medium; - vacuum impedance.

Traditionally, optical phenomena are considered with the help of rays. The description of optical phenomena with the help of rays is called geometric-optical. The rules for finding ray trajectories developed in geometric optics are widely used in practice for the analysis of optical phenomena and in the construction of various optical devices.

Let's give a definition of a beam based on the electromagnetic representation of light waves. First of all, rays are lines along which electromagnetic waves propagate. For this reason Ray is a line, at each point of which the average Poynting vector of an electromagnetic wave is directed tangentially to this line.

In homogeneous isotropic media, the direction of the mean Poynting vector coincides with the normal to the wave surface (equiphase surface), i.e. along the wave vector .

Thus, in homogeneous isotropic media, the rays are perpendicular to the corresponding wavefront of an electromagnetic wave.

For example, consider the rays emitted by a point monochromatic light source. From the point of view of geometric optics, a set of rays emanate from the source point in the radial direction. From the position of the electromagnetic essence of light, a spherical electromagnetic wave propagates from the source point. At a sufficiently large distance from the source, the curvature of the wave front can be neglected, assuming a locally spherical wave to be plane. By dividing the surface of the wave front into a large number of locally flat sections, it is possible to draw a normal through the center of each section, along which the plane wave propagates, i.e. in the geometric-optical interpretation of the beam. Thus, both approaches give the same description of the considered example.

The main task of geometric optics is to find the direction of the beam (trajectory). The trajectory equation is found after solving the variational problem of finding the minimum of the so-called. actions on the desired trajectories. Without going into details of the rigorous formulation and solution of this problem, we can assume that the rays are trajectories with the smallest total optical length. This statement is a consequence of Fermat's principle.

The variational approach to determining the trajectory of rays can also be applied to inhomogeneous media, i.e. such media, in which the refractive index is a function of the coordinates of the points of the medium. If the function describes the shape of the wavefront surface in an inhomogeneous medium, then it can be found based on the solution of the partial differential equation known as eikonal equation, and in analytical mechanics as the equation Hamilton - Jacobi:

Thus, the mathematical basis of the geometric-optical approximation of the electromagnetic theory is made up of various methods for determining the fields of electromagnetic waves on rays, based on the eikonal equation or in some other way. The geometric-optical approximation is widely used in practice in radio electronics to calculate the so-called. quasi-optical systems.

In conclusion, we note that the ability to describe light simultaneously and from wave positions by solving Maxwell's equations and with the help of rays, the direction of which is determined from the Hamilton-Jacobi equations describing the motion of particles, is one of the manifestations of the dualism of light, which, as is known, led to the formulation of the main principles of quantum mechanics.

Vladimir regional
industrial - commercial
lyceum

abstract

Electromagnetic waves

Completed:
student 11 "B" class
Lvov Mikhail
Checked:

Vladimir 2001

1. Introduction ……………………………………………………… 3

2. The concept of a wave and its characteristics……………………………… 4

3. Electromagnetic waves………………………………………… 5

4. Experimental proof of existence
electromagnetic waves………………………………………… 6

5. Density of electromagnetic radiation flux ……………. 7

6. The invention of radio……………………………………………….… 9

7. Properties of electromagnetic waves …………………………………10

8. Modulation and detection…………………………………… 10

9. Types of radio waves and their propagation…………………………… 13

Introduction

Wave processes are extremely widespread in nature. There are two types of waves in nature: mechanical and electromagnetic. Mechanical waves propagate in matter: gas, liquid or solid. Electromagnetic waves do not need any substance for their propagation, which, in particular, include radio waves and light. An electromagnetic field can exist in a vacuum, that is, in a space that does not contain atoms. Despite the significant difference between electromagnetic waves and mechanical waves, electromagnetic waves during their propagation behave like mechanical waves. But like oscillations, all types of waves are described quantitatively by the same or almost the same laws. In my work, I will try to consider the causes of electromagnetic waves, their properties and applications in our lives.

The concept of a wave and its characteristics

wave called vibrations that propagate in space over time.

The most important characteristic of a wave is its speed. Waves of any nature do not propagate through space instantly. Their speed is finite.

When a mechanical wave propagates, motion is transmitted from one part of the body to another. The transfer of motion is associated with the transfer of energy. The main property of all waves, regardless of their nature, is their transfer of energy without the transfer of matter. The energy comes from a source that excites vibrations at the beginning of the cord, string, etc., and propagates along with the wave. Energy flows continuously through any cross section. This energy is composed of the kinetic energy of the movement of the sections of the cord and the potential energy of its elastic deformation. The gradual decrease in the amplitude of oscillations during the propagation of the wave is associated with the transformation of part of the mechanical energy into internal.

If the end of a stretched rubber cord is made to oscillate harmonically with a certain frequency v, then these vibrations will begin to propagate along the cord. Oscillations of any section of the cord occur with the same frequency and amplitude as the oscillations of the end of the cord. But only these oscillations are shifted in phase relative to each other. Such waves are called monochromatic .

If the phase shift between the oscillations of two points of the cord is equal to 2n, then these points oscillate in exactly the same way: after all, cos (2lvt + 2n) \u003d =cos2n vt . Such fluctuations are called in-phase(occur in the same phases).

The distance between points closest to each other, oscillating in the same phases, is called the wavelength.

Relationship between wavelength λ, frequency v and wave propagation speed c. For one period of oscillations, the wave propagates over a distance λ. Therefore, its speed is determined by the formula

Since the period T and frequency v are related by T = 1 / v

The speed of a wave is equal to the product of the wavelength and the oscillation frequency.

Electromagnetic waves

We now turn to the consideration of electromagnetic waves directly.

The fundamental laws of nature can give much more than is contained in the facts on the basis of which they are derived. One of these are the laws of electromagnetism discovered by Maxwell.

Among the countless, very interesting and important consequences arising from the Maxwellian laws of the electromagnetic field, one deserves special attention. This is the conclusion that the electromagnetic interaction propagates at a finite speed.

According to the theory of short-range action, moving a charge changes the electric field near it. This alternating electric field generates an alternating magnetic field in neighboring regions of space. An alternating magnetic field, in turn, generates an alternating electric field, etc.

The movement of the charge thus causes a "burst" of the electromagnetic field, which, while spreading, covers all large areas of the surrounding space.

Maxwell proved mathematically that the propagation speed of this process is equal to the speed of light in vacuum.

Imagine that the electric charge is not just shifted from one point to another, but is brought into rapid oscillations along some straight line. Then the electric field in the immediate vicinity of the charge will begin to change periodically. The period of these changes will obviously be equal to the period of charge oscillations. An alternating electric field will generate a periodically changing magnetic field, and the latter, in turn, will cause the appearance of an alternating electric field already at a greater distance from the charge, etc.

At each point in space, electric and magnetic fields change periodically over time. The farther the point is from the charge, the later its field oscillations will reach. Consequently, at different distances from the charge, oscillations occur with different phases.

The directions of the oscillating vectors of the electric field strength and magnetic field induction are perpendicular to the direction of wave propagation.

The electromagnetic wave is transverse.

Electromagnetic waves are emitted by oscillating charges. It is essential that the speed of movement of such charges varies with time, i.e., that they move with acceleration. The presence of acceleration is the main condition for the radiation of electromagnetic waves. The electromagnetic field is radiated in a noticeable way, not only when the charge fluctuates, but also with any rapid change in its speed. The intensity of the emitted wave is the greater, the greater the acceleration with which the charge moves.

Maxwell was deeply convinced of the reality of electromagnetic waves. But he did not live to see their experimental discovery. Only 10 years after his death, electromagnetic waves were experimentally obtained by Hertz.

Experimental proof of existence

electromagnetic waves

Electromagnetic waves are not visible, unlike mechanical waves, but then how were they detected? To answer this question, consider the experiments of Hertz.

An electromagnetic wave is formed due to the interconnection of alternating electric and magnetic fields. Changing one field leads to the appearance of another. As you know, the faster the magnetic induction changes with time, the greater the strength of the emerging electric field. And in turn, the faster the electric field strength changes, the greater the magnetic induction.

For the formation of intense electromagnetic waves, it is necessary to create electromagnetic oscillations of a sufficiently high frequency.

High frequency oscillations can be obtained using an oscillatory circuit. The oscillation frequency is 1/ √ LC. From here it can be seen that it will be the greater, the smaller the inductance and capacitance of the circuit.

To obtain electromagnetic waves, G. Hertz used a simple device, now called the Hertz vibrator.

This device is an open oscillatory circuit.

It is possible to switch to an open circuit from a closed circuit if the capacitor plates are gradually moved apart, reducing their area and at the same time reducing the number of turns in the coil. In the end, it will just be a straight wire. This is the open oscillatory circuit. The capacitance and inductance of the Hertz vibrator are small. Therefore, the oscillation frequency is very high.

In an open circuit, the charges are not concentrated at the ends, but are distributed throughout the conductor. The current at a given time in all sections of the conductor is directed in the same direction, but the current strength is not the same in different sections of the conductor. At the ends, it is equal to zero, and in the middle it reaches a maximum (in conventional AC circuits, the current strength in all sections is the same at a given time.) The electromagnetic field also covers the entire space near the circuit.

Hertz received electromagnetic waves by exciting a series of fast-alternating current pulses in a vibrator using a high voltage source. Oscillations of electric charges in the vibrator create an electromagnetic wave. Only oscillations in the vibrator are performed not by one charged particle, but by a huge number of electrons moving in concert. In an electromagnetic wave, the vectors E and B are perpendicular to each other. Vector E lies in a plane passing through the vibrator, and vector B is perpendicular to this plane. The radiation of waves occurs with maximum intensity in the direction perpendicular to the axis of the vibrator. There is no radiation along the axis.

Electromagnetic waves were recorded by Hertz using a receiving vibrator (resonator), which is the same device as the radiating vibrator. Under the action of an alternating electric field of an electromagnetic wave, current oscillations are excited in the receiving vibrator. If the natural frequency of the receiving vibrator coincides with the frequency of the electromagnetic wave, resonance is observed. Oscillations in the resonator occur with a large amplitude when it is located parallel to the radiating vibrator. Hertz detected these vibrations by observing sparks in a very small gap between the conductors of the receiving vibrator. Hertz not only obtained electromagnetic waves, but also discovered that they behave like other types of waves.

By calculating the natural frequency of the electromagnetic oscillations of the vibrator. Hertz was able to determine the speed of an electromagnetic wave by the formula c \u003d λ v . It turned out to be approximately equal to the speed of light: c = 300,000 km/s. Hertz's experiments brilliantly confirmed Maxwell's predictions.

Flux density of electromagnetic radiation

Now let's move on to the consideration of the properties and characteristics of electromagnetic waves. One of the characteristics of electromagnetic waves is the density of electromagnetic radiation.

Consider a surface with an area S through which electromagnetic waves carry energy.

The density of the electromagnetic radiation flux I is the ratio of electromagnetic energy Wpassing in time t through a surface perpendicular to the rays with an area S, to the product of the area S and time t.

The radiation flux density, in SI, is expressed in watts per square meter (W / m 2). Sometimes this quantity is called the intensity of the wave.

After a series of transformations, we get that I = w c.

i.e., the density of the radiation flux is equal to the product of the density of electromagnetic energy and the speed of its propagation.

We have met more than once with the idealization of real sources of acceptance in physics: a material point, an ideal gas, etc. Here we will meet with another one.

A radiation source is considered to be a point source if its dimensions are much smaller than the distance at which its effect is estimated. In addition, it is assumed that such a source sends electromagnetic waves in all directions with the same intensity.

Let us consider the dependence of the radiation flux density on the distance to the source.

The energy that electromagnetic waves carry with them is distributed over a larger and larger surface over time. Therefore, the energy transferred through a unit area per unit time, i.e., the radiation flux density, decreases with distance from the source. It is possible to find out the dependence of the radiation flux density on the distance to the source by placing a point source in the center of a sphere with a radius R . the surface area of ​​the sphere S= 4 n R^2. If we assume that the source in all directions during the time t radiates energy W

The radiation flux density from a point source decreases in inverse proportion to the square of the distance to the source.

Let us now consider the frequency dependence of the radiation flux density. As you know, the radiation of electromagnetic waves occurs during the accelerated movement of charged particles. The strength of the electric field and the magnetic induction of an electromagnetic wave are proportional to the acceleration A emitting particles. Harmonic acceleration is proportional to the square of the frequency. Therefore, the electric field strength and magnetic induction are proportional to the square of the frequency

The energy density of the electric field is proportional to the square of the field strength. The energy of the magnetic field is proportional to the square of the magnetic induction. The total energy density of the electromagnetic field is equal to the sum of the energy densities of the electric and magnetic fields. Therefore, the radiation flux density is proportional to: (E^2+B^2). From here we get that I is proportional to w^4.

The radiation flux density is proportional to the fourth power of the frequency.

invention of radio

Hertz's experiments interested physicists all over the world. Scientists began to look for ways to improve the emitter and receiver of electromagnetic waves. In Russia, Alexander Stepanovich Popov, a teacher of officer courses in Kronstadt, was one of the first to study electromagnetic waves.

A. S. Popov used a coherer as a part that directly “feels” electromagnetic waves. This device is a glass tube with two electrodes. Small metal filings are placed in the tube. The operation of the device is based on the effect of electrical discharges on metal powders. Under normal conditions, the coherer has a high resistance, since the sawdust has poor contact with each other. The incoming electromagnetic wave creates a high-frequency alternating current in the coherer. The smallest sparks jump between the sawdust, which sinter the sawdust. As a result, the resistance of the coherer drops sharply (in the experiments of A.S. Popov from 100,000 to 1000-500 ohms, i.e., by a factor of 100-200). You can return the device to high resistance again by shaking it. To ensure the automatic reception necessary for wireless communication, A. S. Popov used a ringing device to shake the coherer after receiving the signal. The electric bell circuit was closed by means of a sensitive relay at the moment of arrival of an electromagnetic wave. With the end of the reception of the wave, the work of the bell immediately stopped, since the hammer of the bell struck not only the bell cup, but also the coherer. With the last shake of the coherer, the apparatus was ready to receive a new wave.

To increase the sensitivity of the device, A. S. Popov grounded one of the coherer leads and connected the other to a highly raised piece of wire, creating the first receiving antenna for wireless communication. Grounding turns the conductive surface of the earth into part of an open oscillatory circuit, which increases the reception range.

Although modern radio receivers bear very little resemblance to A. S. Popov's receiver, the basic principles of their operation are the same as in his device. A modern receiver also has an antenna in which the incoming wave causes very weak electromagnetic oscillations. As in the receiver of A. S. Popov, the energy of these oscillations is not used directly for reception. Weak signals only control the energy sources that feed the subsequent circuits. Now such control is carried out using semiconductor devices.

On May 7, 1895, at a meeting of the Russian Physical and Chemical Society in St. Petersburg, A. S. Popov demonstrated the operation of his device, which was, in fact, the world's first radio receiver. May 7th was the birthday of radio.

Properties of electromagnetic waves

Modern radio engineering devices make it possible to carry out very demonstrative experiments on observing the properties of electromagnetic waves. In this case, it is best to use the waves of the centimeter range. These waves are emitted by a special microwave generator. The electrical oscillations of the generator modulate the sound frequency. The received signal after detection is fed to the loudspeaker.

I will not describe the conduct of all experiments, but will focus on the main ones.

1. Dielectrics are capable of absorbing electromagnetic waves.

2. Some substances (for example, metal) are capable of absorbing electromagnetic waves.

3. Electromagnetic waves are capable of changing their direction at the dielectric boundary.

4. Electromagnetic waves are transverse waves. This means that the vectors E and B of the electromagnetic field of the wave are perpendicular to the direction of its propagation.

Modulation and detection

Since the invention of radio by Popov, some time has passed when people wanted to transmit speech and music instead of telegraph signals, consisting of short and long signals. This is how radiotelephone communication was invented. Consider the basic principles of the operation of such a connection.

In radiotelephone communication, air pressure fluctuations in a sound wave are converted by a microphone into electrical vibrations of the same form. It would seem that if these vibrations are amplified and fed into the antenna, then it will be possible to transmit speech and music over a distance using electromagnetic waves. However, in reality, such a method of transmission is not feasible. The fact is that vibrations of sound of a new frequency are relatively slow vibrations, and electromagnetic waves of low (sound) frequency are almost not emitted at all. To overcome this obstacle, modulation and detection were developed, let's consider them in detail.

Modulation. To carry out radiotelephone communication, it is necessary to use high-frequency vibrations intensely radiated by the antenna. Continuous high-frequency harmonic oscillations are generated by an oscillator, such as a transistor oscillator.

To transmit sound, these high-frequency vibrations are modified, or as they say, modulated, with the help of electrical vibrations of low (sound) frequency. It is possible, for example, to change the amplitude of high-frequency oscillations with sound frequency. This method is called amplitude modulation.

a graph of high frequency oscillations, which is called the carrier frequency;

b) a graph of sound frequency oscillations, i.e., modulating oscillations;

c) a graph of amplitude-modulated oscillations.

Without modulation, at best, we can control whether the station is working or silent. Without modulation, there is no telegraph, telephone, or television transmission.

Amplitude modulation of high-frequency oscillations is achieved by a special effect on the generator of continuous oscillations. In particular, modulation can be carried out by changing the voltage created by the source on the oscillatory circuit. The greater the voltage on the generator circuit, the more energy is supplied per period from the source to the circuit. This leads to an increase in the amplitude of oscillations in the circuit. When the voltage decreases, the energy entering the circuit also decreases. Therefore, the amplitude of oscillations in the circuit also decreases.

In the simplest device for amplitude modulation, an additional source of low-frequency alternating voltage is connected in series with the DC voltage source. This source can be, for example, the secondary winding of a transformer, if an audio frequency current flows through its primary winding. As a result, the amplitude of oscillations in the oscillatory circuit of the generator will change in time with changes in the voltage across the transistor. This means that high-frequency oscillations are modulated in amplitude by a low-frequency signal.

In addition to amplitude modulation, in some cases frequency modulation is used - a change in the oscillation frequency in accordance with the control signal. Its advantage is greater resistance to interference.

Detection. In the receiver, low-frequency oscillations are distinguished from the modulated high-frequency oscillations. This signal conversion process is called detection.

The signal obtained as a result of detection corresponds to the sound signal that acted on the transmitter microphone. After amplification, low frequency vibrations can be turned into sound.

The modulated high-frequency signal received by the receiver, even after amplification, is not capable of directly causing oscillations of the telephone membrane or the horn of the loudspeaker with an audio frequency. It can only cause high-frequency vibrations that are not perceived by our ear. Therefore, in the receiver, it is first necessary to isolate the audio frequency signal from high-frequency modulated oscillations.

Detection is carried out by a device containing an element with one-way conduction - a detector. Such an element can be a vacuum tube (vacuum diode) or a semiconductor diode.

Consider the operation of a semiconductor detector. Let this device be connected in series with the source of modulated oscillations and the load. The current in the circuit will flow predominantly in one direction.

A pulsating current will flow in the circuit. This pulsating current is smoothed out by a filter. The simplest filter is a capacitor connected to a load.

The filter works like this. At those moments in time when the diode passes current, part of it passes through the load, and the other part branches into the capacitor, charging it. Current splitting reduces the ripple of the current passing through the load. But in the interval between pulses, when the diode is locked, the capacitor is partially discharged through the load.

Therefore, in the interval between pulses, the current flows through the load in the same direction. Each new pulse recharges the capacitor. As a result, an audio-frequency current flows through the load, the waveform of which almost exactly reproduces the waveform of the low-frequency signal at the transmitting station.

Types of radio waves and their propagation

We have already considered the basic properties of electromagnetic waves, their application in radio, the formation of radio waves. Now let's get acquainted with the types of radio waves and their propagation.

The shape and physical properties of the earth's surface, as well as the state of the atmosphere, greatly affect the propagation of radio waves.

A particularly significant effect on the propagation of radio waves is exerted by layers of ionized gas in upper parts atmosphere at an altitude of 100-300 km above the Earth's surface. These layers are called the ionosphere. The ionization of the air of the upper layers of the atmosphere is caused by the electromagnetic radiation of the Sun and the flow of charged particles emitted by it.

The electrically conductive ionosphere reflects radio waves with a wavelength > 10 m, like an ordinary metal plate. But the ability of the ionosphere to reflect and absorb radio waves varies significantly depending on the time of day and seasons.

Stable radio communication between remote points on the earth's surface outside the line of sight is possible due to the reflection of waves from the ionosphere and the ability of radio waves to bend around the convex earth's surface. This bending is more pronounced, the longer the wavelength. Therefore, radio communication over long distances due to wave bending around the Earth is possible only at wavelengths significantly exceeding 100 m ( medium and long waves)

short waves(wavelength range from 10 to 100 m) propagate over long distances only due to multiple reflections from the ionosphere and the Earth's surface. It is with the help of short waves that radio communication can be carried out at any distance between radio stations on Earth.

ultrashort radio waves (λ <10 м) проникают сквозь ионосферу и почти не огибают поверхность Земли. Поэтому они используются для радиосвязи между пунктами в пределах прямой видимости, а также для связи с космическими кораб­лями.

Now consider another application of radio waves. This is radar.

The detection and precise location of objects using radio waves is called radar. Radar installation - radar(or radar) - consists of transmitting and receiving parts. Radar uses ultra-high frequency electrical vibrations. A powerful microwave generator is connected to an antenna that emits a highly directional wave. The sharp directivity of the radiation is obtained due to the addition of waves. The antenna is designed so that the waves sent by each of the vibrators, when added, mutually reinforce each other only in a given direction. In other directions, when the waves are added, their complete or partial mutual damping occurs.

The reflected wave is captured by the same transmitting antenna or by another, also highly directional receiving antenna.

To determine the distance to the target, a pulsed radiation mode is used. The transmitter emits waves in short pulses. The duration of each pulse is millionths of a second, and the interval between pulses is about 1000 times longer. During pauses, reflected waves are received.

Distance is determined by measuring the total travel time of radio waves to and from the target. Since the speed of radio waves c \u003d 3 * 10 8 m / s in the atmosphere is practically constant, then R \u003d ct / 2.

To fix the sent and reflected signals, a cathode ray tube is used.

Radio waves are used not only to transmit sound, but also to transmit images (television).

The principle of transmitting images over a distance is as follows. At the transmitting station, the image is converted into a sequence of electrical signals. These signals then modulate the oscillations generated by the high frequency generator. A modulated electromagnetic wave carries information over long distances. The receiver performs the reverse conversion. High-frequency modulated oscillations are detected and the received signal is converted into a visible image. To convey movement, the principle of cinema is used: slightly different images of a moving object (frames) are transmitted dozens of times per second (50 times in our television).

The image of the frame is converted by a transmitting vacuum electron tube - an iconoscope into a series of electrical signals. In addition to the iconoscope, there are other transmitting devices. Inside the iconoscope there is a mosaic screen onto which an image of the object is projected with the help of an optical system. Each cell of the mosaic is charged, and its charge depends on the intensity of the light falling on the cell. This charge changes when the electron beam produced by the electron gun hits the cell. The electron beam sequentially hits all elements, first of one line of the mosaic, then another line, etc. (625 lines in total).

How much the charge of the cell changes depends on the current strength in the resistor R . Therefore, the voltage across the resistor changes in proportion to the change in illumination along the lines of the frame.

The same signal is obtained in the television receiver after detection. This video signal. It is converted into a visible image on the screen of the receiving vacuum electron tube - kinescope.

Television radio signals can only be transmitted in the range of ultrashort (meter) waves.

Bibliography.

1. Myakishev G.Ya. , Bukhovtsev B.B. Physics - 11. M. 1993.

2. Telesnin R.V., Yakovlev V.F. Physics course. Electricity. M. 1970

3. Yavorsky B.M., Pinsky A.A. Fundamentals of physics. v. 2. M. 1981