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CHILDREN'S SCIENTIFIC LABORATORY
The red sun will rise. Children's Science Lab
Directory / Children's Science Lab The color of the sky, the color of the Sun and the Moon, many optical and acoustic phenomena are determined by the fact that electromagnetic and elastic waves of different lengths are scattered in the atmosphere in different ways, obeying the Rayleigh law. In summer, few people see the rising sun - it rises too early. But the sunsets appear before us in all their glory: a huge ball, changing its color from bright red to maroon, slowly descends across the blue sky, coloring it in yellow, green, pink tones, and disappears beyond the horizon... When- It was believed that the air itself has a blue color and therefore the atmosphere absorbs red rays. But then the Sun and the Moon at the horizon would seem more bluish than at the zenith: the rays of light from them, before reaching the observer, pass through the greater the thickness of the air, the lower the luminary descends. After the advent of the electromagnetic theory of light, it became clear that light waves in the atmosphere must be scattered by particles suspended in the air, like waves on water - stones and rocks that stand in their way. This was suggested and proved experimentally in 1868 by the English physicist J. Tyndall. However, three years later, J.W. Rayleigh showed that light scattering should also occur in an ideally clean atmosphere on its optical inhomogeneities - density fluctuations. These inhomogeneities continuously arise as a result of a random accumulation of molecules during their thermal motion and instantly dissolve in order to form again in another place. Light passing through a void or through an absolutely homogeneous medium does not scatter: the dimensions of the molecules are thousands of times smaller than the wavelength of light, and the light travels without "noticing" them. The inhomogeneities of the medium become a kind of prisms, which scatter light the more strongly, the more the air density in them differs from the average value. And, of course, the more such inhomogeneities. A medium with optical inhomogeneities measuring 0,1-0,2 of the average wavelength of light is called turbid. In a turbid medium, light waves of different lengths scatter differently: short-wave radiation, the blue part of the spectrum is stronger, long-wave, red is weaker. The dependence of scattering on the wavelength is very strong - it is inversely proportional to the fourth power of the wavelength. This means that blue light, the wavelength of which (0,5 μm) is 1,4 times smaller than the wavelength of red light (0,7 μm), is scattered in a turbid medium in (1,4)4=4 times stronger!
An electromagnetic wave, falling on the molecules of a substance, interacts with their electrons. Electrons so weakly bound to atoms that they can be noticeably displaced by the action of the wave (they are therefore called "optical electrons") experience a periodic acceleration proportional to the square of the frequency, and generate an alternating magnetic field. A secondary electromagnetic wave arises in the field, the amplitude of which is proportional to the acceleration of the electron, and the intensity is proportional to the square of the amplitude. Thus, the intensity of the emitted secondary light is proportional to the fourth power of the frequency of the incident light, or - which is the same - inversely proportional to the fourth power of its wavelength. This secondary radiation is the light scattered in a turbid medium, and the dependence of its intensity on the wavelength is called Rayleigh's law.
Particles larger than the wavelength of light (0,5-0,7 μm) scatter light mainly in the direction of the incident beam, and the distribution of its intensity becomes quite complex.
Particles with a size of about 0,1 μm scatter the incident light equally forward and backward and in the transverse direction is twice as weak as in the longitudinal direction. This relationship is called Rayleigh's law. It explains the red color of the setting sun, the blue color of the sky, and the color of sea water (in shallow water, yellow, reflected from the sandy bottom, is added to the blue diffused light, and the water turns green). For the same reason, warning lights, brake lights and other danger signs are made red (they can be seen from afar), and a red filter on the camera lens helps when shooting in haze. In such pictures, the sky is very dark, almost black, the foliage is light, and the details of distant objects come out quite clearly. (Note in passing that photographers and cinematographers use a red filter to depict a moonlit night when shooting on a bright sunny afternoon.) The blue filter, on the contrary, creates a sense of a mysterious world hidden behind a foggy veil in the picture. During the war, the entrances of houses were illuminated with blue pampas - their light, quickly dissipating in the atmosphere, was not visible from the air. Very small particles scatter light equally strongly along the incident beam and against it, and 2 times weaker - in the perpendicular direction. The color saturation of the sky also changes accordingly. When the particles become larger, this dependence becomes much more complex. The light begins to scatter mainly forward, in the direction of the incident light, and its spectral composition also changes. The dependence on the wavelength becomes not Rapey (Lambda4), but quadratic (Lambda2). As they become even larger, the particles begin to scatter all wavelengths equally. This happens when a light haze thickens and turns into a milky white fog. For this reason, yellow-orange "fog" car lights do not really work in fog: their light is scattered there as much as white. Moreover: in a strong haze, it becomes reddish, and it can be confused with the rear lights of a receding car (sometimes with the most unfortunate consequences). In the steppes and deserts, a whitish sky is an alarming sign. He says that a strong wind is coming, a hurricane that raises clouds of fine sand and dust into the air. And only rain, "washing" the air, can return the sky blue. The sign is also fair: "The moon turns red - to the wind and bad weather." The wind intensively mixes layers of air of different temperatures; the number of fluctuations increases sharply in this case. By setting up a simple experiment, you can see how the colors of transmitted and scattered light change (see figure.). A weak solution of hyposulfite is poured into a glass jar. A beam of white light from a slide projector is passed through a vessel and focused on a paper screen to form a circle of light. Then dilute hydrochloric acid is added dropwise to the jar (the concentration of solutions is selected empirically). After a few minutes, the reaction product, finely dispersed sulfur, will begin to precipitate from the solution. The sulfur particles increase in size, and at the same time the light spot on the screen turns first yellow, then red, and finally crimson, reminiscent of the setting sun. The solution in the vessel, which was completely transparent at the beginning of the experiment, acquires a blue color, which eventually becomes whitish, like fog. If you wait until the sulfur particles settle to the bottom, the solution will again become transparent, and the light spot will become white. Sound waves and waves on water behave in a similar way: their low frequencies are also scattered much weaker than high ones. Sound vibrations interact with the medium in a completely different way than electromagnetic vibrations - they "swing" not individual electrons in air molecules, but entire areas of increased density and particles suspended in it. Fog dissipates and absorbs sound especially strongly. Sounds in the fog become muffled, low, and it is difficult to determine where they are coming from. Interesting things sometimes happen with sound reflected from distant objects - an echo. J. Rayleigh investigated the case when the sound of a voice reflected from the wall of a pine forest rose by an octave. It is quite obvious that the frequency of sound vibrations cannot increase only due to reflection from an immovable obstacle. But the human voice, in addition to the fundamental tone, contains many additional overtones of a higher frequency, which we usually do not perceive. Pine trees, with their thin and sparse needles, serve as a "muddy medium" for sound, which transmits low frequencies well, and reflects high ones. Only the overtones of his voice return to the observer, and it seems that the whole sound suddenly became higher. People with heightened creative perception - writers, poets, composers - are well aware of this feature of atmospheric acoustics. In A.P. Chekhov's story "Doctor" there is a remarkable phrase: "At this time, the sounds of an orchestra playing in the dacha circle were distinctly heard from the yard. Not only trumpets, but even violins and flutes were heard." In the open air, the flute and violin can really be heard from afar only in especially favorable conditions. And the composers, depicting the outgoing military orchestra, do not just reduce the volume of its sound, but first of all gradually remove all high sounds. The music becomes quieter, the melody gradually disappears, and only the muffled beats of the bass drum and the fading sighs of the bass helicon remain. The regiment is gone... The red sun is rising... White light changes color
Many of the optical phenomena we see on a daily basis are due to the fact that light of different wavelengths scatters differently along its path. The sun near the horizon - at sunrise and sunset - is always red. The evening sky is blue or blue very rarely - only when the air in the surface layer is completely free from dust and moisture. The colors of dawn create, mixing, light waves of different lengths scattered in a dusty atmosphere. The milk ball of the lamp on the escalator of the Mayakovskaya metro station and the frosted cap of the table lamp. Milky glass, containing an extremely fine opaque dye, serves as a "muddy medium" for light, strongly scattering the short-wavelength part of the spectrum. A white-hot lamp filament therefore appears dark red. Rough scratches on the ground glass scatter electromagnetic waves of any length equally, and the entire lamp cover glows with white light.
Author: S.Trankovsky
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