We are all accustomed to the fact that the color of the sky is a fickle characteristic. Fog, clouds, time of day - everything affects the color of the dome over your head. Its daily change does not occupy the minds of most adults, which cannot be said about children. They are constantly wondering why the sky is blue in terms of physics, or what colors the sunset red. Let's try to understand these not the easiest questions.
Changeable
It is worth starting with an answer to the question of what, in fact, the sky is. V ancient world it really was seen as a dome covering the Earth. Today, however, hardly anyone does not know that, no matter how high the curious explorer ascends, he will not be able to reach this dome. The sky is not a thing, but rather a panorama that opens when viewed from the surface of the planet, a kind of appearance woven of light. Moreover, if you observe from different points, it may look different. So, from the one that has risen above the clouds, a completely different view opens than from the ground at this time.
The clear sky is blue, but as soon as the clouds come in, it turns gray, leaden, or off-white. The night sky is black, sometimes you can see reddish areas on it. This is a reflection of the city's artificial lighting. The reason for all such changes is light and its interaction with air and particles of various substances in it.
The nature of color
In order to answer the question of why the sky is blue from the point of view of physics, you need to remember what color is. This is a wave of a certain length. The light coming from the Sun to the Earth is seen as white. Even from Newton's experiments, it is known that it is a bundle of seven rays: red, orange, yellow, green, light blue, blue and violet. Colors differ in wavelength. The red-orange spectrum includes the most impressive waves in this parameter. parts of the spectrum are characterized by a short wavelength. The decomposition of light into a spectrum occurs when it collides with molecules of various substances, while part of the waves can be absorbed, and part of it can be scattered.
Investigating the cause
Many scientists have tried to explain why the sky is blue in terms of physics. All researchers sought to find a phenomenon or process that scatters light in the planet's atmosphere in such a way that as a result, only blue reaches us. Water was also the first candidate for the role of such particles. It was believed that they absorb red light and transmit blue, and as a result, we see the blue sky. Subsequent calculations, however, showed that the amount of ozone, ice crystals and water vapor molecules in the atmosphere was not enough to give the sky a blue color.
The reason is pollution
At the next stage of research, John Tyndall suggested that dust plays the role of the desired particles. Blue light has the greatest resistance to scattering, and therefore is able to pass through all layers of dust and other suspended particles. Tyndall conducted an experiment that confirmed his assumption. He created a model of a smog in a laboratory and illuminated it with bright white light. The smog took on a blue hue. The scientist made an unambiguous conclusion from his research: the color of the sky is determined by dust particles, that is, if the air of the Earth were clean, then not blue, but white skies shone above the heads of people.
Lord's research
The final point on the question of why the sky is blue (from the point of view of physics) was put by the English scientist, Lord D. Rayleigh. He proved that it is not dust or smog that colors the space overhead in the shade we are used to. It's about the air itself. Gas molecules absorb the largest and primarily the longest wavelengths, equivalent to red. In this case, the blue is scattered. This is how the color of the sky we see in clear weather is explained today.
The attentive will notice that, following the logic of scientists, the dome over your head should be purple, since it is this color that has the shortest wavelength in the visible range. However, this is not a mistake: the proportion of violet in the spectrum is much less than that of blue, and human eyes are more sensitive to the latter. In fact, the blue we see is the result of mixing blue with purple and some other colors.
Sunsets and clouds
Everyone knows that in different time day you can see a different color of the sky. Photos of beautiful sunsets over the sea or lake are a great illustration of this. All sorts of shades of red and yellow combined with blue and dark blue make such a spectacle unforgettable. And it is explained by the same scattering of light. The fact is that during dusk and dawn, the sun's rays have to overcome a much larger path through the atmosphere than during the height of the day. In this case, the light of the blue-green part of the spectrum is scattered in different directions and the clouds located at the horizon line become colored in shades of red.
When the sky is covered with clouds, the picture changes completely. unable to overcome the dense layer, and most of they simply do not reach the ground. The rays that have managed to pass through the clouds meet with water drops of rain and clouds, which again distort the light. As a result of all these transformations, the earth reaches White light if the clouds are small in size, and gray when the sky is covered by impressive clouds, which again absorb part of the rays.
Other heaven
I wonder what on other planets Solar system when viewed from the surface, you can see the sky, which is very different from the earth. On space objects, deprived of the atmosphere, the sun's rays freely reach the surface. As a result, the sky here is black, without any shade. Such a picture can be seen on the Moon, Mercury and Pluto.
The Martian sky has a reddish-orange hue. The reason for this lies in the dust with which the atmosphere of the planet is saturated. It is painted in different shades of red and orange. When the Sun rises above the horizon, the Martian sky turns pinkish-red, while the portion of it immediately surrounding the luminary's disk appears blue or even purple.
The sky above Saturn is the same color as on Earth. Aquamarine skies stretch over Uranus. The reason lies in the methane haze located in the upper planets.
Venus is hidden from the eyes of researchers by a dense layer of clouds. It does not allow the rays of the blue-green spectrum to reach the planet's surface, so the sky here is yellow-orange with a gray stripe along the horizon.
Exploring overhead space during the day reveals no less wonders than exploring the starry sky. Understanding the processes taking place in and behind the clouds helps to comprehend the reason for things that are quite familiar to the average person, which, nevertheless, not everyone can explain right off the bat.
But how many different colors are there that makes the things around us colored? AND scientific knowledge many of these questions can already be answered. For example, explain sky color.
To begin with, it will be necessary to mention the great Isaac Newton, who observed the decomposition of the white solar as it passed through a glass prism. What he saw is now called a phenomenon variance, and the multi-colored picture itself - spectrum... The resulting colors matched exactly the colors of the rainbow. That is, Newton observed a rainbow in the laboratory! It was thanks to his experiments at the end of the 18th century that it was established that white light is a mixture of different colors. Moreover, the same Newton proved that if the light decomposed into a spectrum is mixed again, then white light will turn out. In the 19th century, it was shown that light is propagating at a tremendous speed of 300,000 km / s electromagnetic waves... And already at the beginning of the last century, this knowledge was supplemented with the idea of a quantum of light - photon... Thus, light has a dual nature - both waves and particles. This combination became the explanation of many phenomena, in particular, the spectrum of thermal radiation of heated bodies. Such as ours is.
After this introduction, it's time to move on to our topic. The blue color of the sky ... Who has not admired him at least a couple of times in his life! But is it so easy to say that the scattering of light in the atmosphere is to blame? And why, then, the color of the sky is not blue in the light of the full moon? Why is the blue color not the same in all parts of the sky? What happens to the color of the sky when the sun rises and sets? After all, it can be yellow, pink and even green. But still, these are features of scattering. Therefore, we will consider it in more detail.
The explanation of the color of the sky and its features belongs to the English physicist John William Rayleigh, who studied light scattering. It was he who pointed out that the color of the sky is determined by the dependence of scattering on the frequency of light. Radiation from the Sun, falling into, interacts with gas molecules that make up the air. And since the energy of a light quantum - a photon - increases with decreasing light wavelength, the strongest effect on gas molecules, more precisely, on the electrons in these molecules, is exerted by photons of the blue and violet parts of the light spectrum. Having come into forced oscillations, the electrons give back the energy taken from the light wave in the form of a radiation photon. Only these secondary photons are already being emitted in all directions, not just in the direction of the originally incident light. This will be the process of light scattering. In addition, you need to take into account and constant movement air, and fluctuations in its density. Otherwise, we would be watching a black sky.
And now let's return to the thermal radiation of bodies. The energy in its spectrum is unevenly distributed and is described on the basis of the laws established by the German physicist Wilhelm Wien. The spectrum of our Sun will be just as uneven behind the photon energies. That is, the photons of the violet part of it will be much less than the photons from the blue and the more so blue. If we also take into account the physiology of vision, namely the maximum sensitivity of our eyes to blue-green color, then we end up with a sky of blue or blue color.
It should be taken into account that the longer the path of the sunbeam in the atmosphere, the less non-interacting photons from the blue and blue spectral regions remain in it. Therefore, the color of the sky is uneven, and the morning or evening colors are yellow-red due to the long path of light through the atmosphere. In addition, dust, smoke, and other particles in the air also strongly affect the scattering of light in the atmosphere. On this topic, one can recall the famous London paintings. Or memories of the catastrophe of 1883, which happened during the eruption of the Krakatoa volcano. The ash from the eruption, which entered the atmosphere, caused the bluish color of the sun in many countries of the Pacific region, as well as red dawns observed throughout the Earth. But these effects are already explained by another theory - the theory of scattering by particles comparable with the length of a light wave. This theory was proposed to the world by the German physicist Gustav Mee. Its main idea is that such particles, due to their relatively large size, scatter red light more strongly than blue or violet.
Thus, the color of the sky is not just a source of inspiration for poets and artists, but a consequence of subtle physical laws that human genius managed to reveal.
The joy to see and understand
is the most beautiful gift of nature.
Albert Einstein
The Riddle of Heavenly Blue
Why the sky is blue?...
There is no such person who has not thought about this at least once in his life. Medieval thinkers have already tried to explain the origin of the color of the sky. Some of them suggested that blue is the true color of air or some of its constituent gases. Others thought that the real color of the sky is black - the way it looks at night. During the day, the black color of the sky is combined with white - the sun's rays, and it turns out ... blue.
Now, perhaps, you will not meet a person who, wanting to get blue paint, would mix black and white. And there was a time when the laws of color mixing were still unclear. They were installed only three hundred years ago by Newton.
Newton also became interested in the secret of heavenly blue. He began by rejecting all previous theories.
First, he argued, a mixture of white and black never forms blue. Secondly, blue is not at all the true color of air. If this were so, then the Sun and Moon at sunset would not appear red, as it is in reality, but blue. The peaks of the distant snowy mountains would have looked like this.
Imagine that the air is colored. Even if it is very weak. Then a thick layer of it would act like colored glass. And if you look through tinted glass, then all objects will appear the same color as this glass. Why do distant snowy peaks appear pink to us, and not at all blue?
In a dispute with his predecessors, the truth was on Newton's side. He proved that the air is not colored.
Still, he did not solve the riddle of the blue sky. He was confused by the rainbow, one of the most beautiful, poetic phenomena of nature. Why does it suddenly appear and just as suddenly disappear? Newton could not be satisfied with the prevailing superstition: a rainbow is a sign from above, it portends good weather. He strove to find the material cause of every phenomenon. He also found the cause of the rainbow.
A rainbow is the result of the refraction of light in raindrops. Realizing this, Newton was able to calculate the shape of the rainbow arc and explain the sequence of colors in the rainbow. His theory could not explain only the appearance of a double rainbow, but it was possible to do this only three centuries later with the help of a very complex theory.
The success of the rainbow theory hypnotized Newton. He mistakenly assumed that the blue sky and the rainbow were caused by the same cause. A rainbow does flare up when the sun's rays break through a swarm of raindrops. But the blue sky is visible not only in the rain! On the contrary, it is in clear weather, when there is not even a hint of rain, that the sky is especially blue. How did the great scientist not notice this? Newton thought that the smallest water bubbles, which, according to his theory, form only the blue part of the rainbow, float in the air in any weather. But this was a delusion.
First solution
Almost 200 years have passed, and another English scientist, Rayleigh, took up this issue, not afraid that the task was beyond the power of even the great Newton.
Rayleigh worked in optics. And people who have devoted their lives to the study of light spend a lot of time in the dark. Extraneous light interferes with the finest experiments; therefore, the windows of the optical laboratory are almost always covered with black, impenetrable curtains.
Rayleigh spent hours in his gloomy laboratory alone with beams of light erupting from his instruments. In the path of the rays, they circled like living dust particles. They were brightly lit and therefore stood out against the dark background. The scientist, perhaps, for a long time in thought, followed their smooth movements, just like a man watches the play of sparks in a fireplace.
Was it not these specks of dust dancing in the rays of light that suggested Rayleigh a new idea about the origin of the color of the sky?
Also in deep antiquity it became known that light propagates in a straight line. This important discovery could have been made by a primitive man, observing how, breaking through the cracks of the hut, the sun's rays fall on the walls and floor.
But he was hardly bothered by the thought why he sees light rays, looking at them from the side. And here there is something to think about. After all, sunlight is a ray from the gap to the floor. The eye of the observer is located to the side and, nevertheless, sees this light.
We also see the light from a searchlight directed into the sky. This means that part of the light somehow deviates from the direct path and is sent to our eye.
What makes him go astray? It turns out that the very specks of dust with which the air is full. Rays scattered by a speck of dust enter our eye, which, meeting obstacles, turn off the road and spread in a straight line from the scattering speck of dust to our eye.
"Isn't it these specks of dust that color the sky blue?" Rayleigh once thought. He did the math, and the guess turned into confidence. He found an explanation for the blue sky, red dawns and blue haze! Of course, the smallest specks of dust, the size of which is less than the wavelength of light, scatter sunlight and the more, the shorter its wavelength, Rayleigh announced in 1871. And since the violet and blue rays in the visible solar spectrum have the shortest wavelength, they are scattered most strongly, giving the sky a blue color.
This calculation of Rayleigh obeyed the sun and snowy peaks. They even confirmed the scientist's theory. At sunrise and sunset, when sunlight passes through the greatest thickness of the air, the violet and blue rays, says Rayleigh's theory, are scattered most strongly. At the same time, they deviate from the direct path and do not fall into the eyes of the observer. The observer sees mainly red rays, which are scattered much weaker. Therefore, at sunrise and sunset, the sun appears to us red. For the same reason, the peaks of the distant snowy mountains also appear pink.
Looking at the clear sky, we see blue-blue rays deviating from a straight path due to scattering and falling into our eyes. And the haze that we sometimes see on the horizon also seems to us blue.
An annoying trifle
Nice explanation, isn't it? He was so carried away by Rayleigh himself, scientists were so amazed at the harmony of the theory and Rayleigh's victory over Newton that none of them noticed one simple thing. And this trifle, however, should have completely changed their assessment.
Who would deny that far from the city, where there is much less dust in the air, the blue color of the sky is especially clear and bright? It was hard to deny this to Rayleigh himself. So ... don't the dust particles scatter the light? What then?
He again revised all his calculations and made sure that his equations are correct, but this means that the scattering particles are really not dust particles. In addition, the dust particles that are present in the air are much larger than the wavelength of light, and calculations convinced Rayleigh that a large accumulation of them does not increase the blueness of the sky, but, on the contrary, weakens it. The scattering of light by large particles weakly depends on the wavelength and therefore does not cause a change in its color.
When light is scattered by large particles, both the scattered and transmitted light remains white, therefore, the appearance of large particles in the air gives the sky a whitish color, and the cluster a large number large droplets are responsible for the white color of clouds and fog. It is easy to check this on an ordinary cigarette. The smoke coming out of it from the side of the mouthpiece always seems whitish, and the smoke rising from its burning end has a bluish color.
The smallest particles of smoke rising above the burning end of a cigarette are smaller than the wavelength of light, and in accordance with Rayleigh's theory, they scatter primarily violet and blue. But when passing through narrow channels in the thickness of the tobacco, the smoke particles stick together (coagulate), combining into larger lumps. Many of them become larger than the wavelengths of light, and they scatter all waves of light in about the same way. That is why the smoke coming from the side of the mouthpiece appears whitish.
Yes, arguing and defending the theory based on dust particles was useless.
So, the mystery of the blue color of the sky again appeared before the scientists. But Rayleigh didn't give up. If the blue color of the sky is the purer and brighter the cleaner the atmosphere, he reasoned, then the color of the sky cannot be caused by anything other than the molecules of the air itself. Air molecules, he wrote in his new articles, are the smallest particles that scatter the light of the sun!
Rayleigh was very careful this time. Before announcing his new idea, he decided to test it, somehow check the theory against experience.
The case presented itself in 1906. Rayleigh was helped by the American astrophysicist Abbot, who studied the blue glow of the sky at the Mount Wilson Observatory. By processing the results of measuring the brightness of the glow of the sky based on Rayleigh's scattering theory, Abbot calculated the number of molecules contained in each cubic centimeter of air. It turned out to be a grandiose number! Suffice it to say that if you distribute these molecules to all people inhabiting the globe, then everyone will get more than 10 billion of these molecules. In short, Abbot found that in every cubic centimeter of air at normal temperature and pressure of the atmosphere, there are 27 billion times a billion molecules.
The number of molecules in a cubic centimeter of gas can be determined different ways on the basis of completely different and independent phenomena. All of them lead to closely coinciding results and give a number called the Loschmidt number.
This number is well known to scientists, and more than once it served as a measure and control in explaining the phenomena that occur in gases.
And now the number obtained by Abbott when measuring the glow of the sky coincided with great accuracy with the number of Loschmidt. But he used Rayleigh's scattering theory in his calculations. Thus, it clearly proved that the theory is correct, molecular scattering of light really exists.
It seemed that Rayleigh's theory was reliably confirmed by experience; all scientists considered her to be impeccable.
It became generally recognized and entered all optics textbooks. One could breathe calmly: an explanation of the phenomenon has finally been found - so familiar and at the same time mysterious.
It is all the more surprising that in 1907, on the pages of a well-known scientific journal, the question was again raised: why is the sky blue ?!
Dispute
Who dared to question the generally accepted Rayleigh theory?
Ironically, this was one of Rayleigh's ardent fans and admirers. Perhaps no one appreciated and understood Rayleigh so well, did not know his work so well, was not interested in his scientific work as much as the young Russian physicist Leonid Mandelstam.
- In the nature of Leonid Isaakovich's mind, - later recalled another Soviet scientist, Academician N.D. Papaleksi - had a lot in common with Rayleigh. And it is no coincidence that the paths of their scientific work often went in parallel and repeatedly crossed.
They crossed themselves this time too, on the question of the origin of the color of the sky. Prior to that, Mandelstam was mainly fond of radio engineering. For the beginning of our century, this was a completely new field of science, and few people understood it. After A.S. Popov (in 1895) only a few years passed, and there was a lot of work here. In a short period of time, Mandelstam carried out a lot of serious research in the field of electromagnetic oscillations as applied to radio engineering devices. In 1902 he defended his dissertation and at twenty-three received his Ph.D. in natural philosophy from the University of Strasbourg.
While dealing with the excitation of radio waves, Mandelstam naturally studied the works of Rayleigh, who was a recognized authority in the study of oscillatory processes. And the young doctor involuntarily got acquainted with the problem of the color of the sky.
But, having become acquainted with the issue of the color of the sky, Mandelstam not only showed the fallacy, or, as he himself said, the "insufficiency" of the generally accepted theory of molecular scattering of light by Rayleigh, not only revealed the secret of the blue color of the sky, but also initiated research that led to one of the most important discoveries of physics of the XX century.
It all started with a correspondence dispute with one of the greatest physicists, the father of quantum theory, M. Planck. When Mandelstam became acquainted with Rayleigh's theory, she captured him with her reticence and internal paradoxes, which, to the surprise of the young physicist, the old, experienced Rayleigh did not notice. The inadequacy of Rayleigh's theory was especially clearly revealed when analyzing another theory based on it by Planck to explain the attenuation of light when it passes through an optically homogeneous transparent medium.
In this theory, it was taken as a basis that the very molecules of the substance through which the light passes are sources of secondary waves. To create these secondary waves, Planck argued, part of the energy of the passing wave is spent, which is then weakened. We see that this theory is based on the Rayleigh theory of molecular scattering and relies on its authority.
The easiest way to understand the essence of the matter is by examining the waves on the surface of the water. If a wave meets fixed or floating objects (piles, logs, boats, etc.), then small waves scatter in all directions from these objects. This is nothing more than scattering. Part of the incident wave energy is spent on the excitation of secondary waves, which are quite analogous to scattered light in optics. In this case, the initial wave is weakened - it is attenuated.
Floating objects can be much shorter than the wavelength of the water traveling. Even small grains will cause secondary waves. Of course, as the size of the particles decreases, the secondary waves generated by them will weaken, but they will still take up the energy of the main wave.
This is approximately how Planck imagined the process of weakening a light wave when it passes through a gas, but the role of grains in his theory was played by gas molecules.
Mandelstam became interested in this work.
Mandelstam's train of thought can also be explained using the example of waves on the surface of the water. You just need to consider it more closely. So, even small grains floating on the surface of the water are sources of secondary waves. But what happens if these grains are poured so thick that they cover the entire surface of the water? Then it turns out that individual secondary waves caused by numerous grains will fold in such a way that they completely extinguish those parts of the waves that run to the sides and back, and the scattering will stop. There will be only a wave running forward. She will run forward, not weakening at all. The only result of the presence of the entire mass of grains will be a slight decrease in the speed of propagation of the primary wave. It is especially important that all this does not depend on whether the grains are stationary or they are moving along the surface of the water. The aggregate of grains will simply act as a load on the surface of the water, changing the density of its upper layer.
Mandelstam performed a mathematical calculation for the case when the number of molecules in the air is so large that even in such a small area as the length of a light wave, a very large number of molecules are contained. It turned out that in this case the secondary light waves, excited by individual chaotically moving molecules, add up in the same way as waves in the example with grains. This means that in this case the light wave propagates without scattering and attenuation, but at a slightly lower speed. This refuted the theory of Rayleigh, who believed that the motion of scattering particles in all cases ensures the scattering of waves, and therefore refuted the Planck theory based on it.
So, sand was discovered under the foundation of the theory of scattering. The entire majestic building shook and threatened to collapse.
Coincidence
But what about the determination of the Loschmidt number from measurements of the blue glow of the sky? After all, experience confirmed the Rayleigh theory of scattering!
"This coincidence should be regarded as accidental," wrote Mandelstam in 1907 in his work "On Optically Homogeneous and Turbid Media."
Mandelstam showed that the disordered movement of molecules cannot make a gas homogeneous. On the contrary, in a real gas there are always the smallest rarefaction and condensation, which are formed as a result of a chaotic thermal motion... It is they who lead to the scattering of light, since they violate the optical homogeneity of the air. In the same work, Mandelstam wrote:
"If the medium is optically inhomogeneous, then, generally speaking, the incident light will be scattered to the sides."
But since the dimensions of the inhomogeneities resulting from chaotic motion are less than the length of light waves, the waves corresponding to the violet and blue parts of the spectrum will be scattered mainly. And this leads, in particular, to the blue color of the sky.
So the riddle of the sky blue was finally solved. The theoretical part was developed by Rayleigh. The physical nature of the scatterers was established by Mandelstam.
The great merit of Mandelstam lies in the fact that he proved that the assumption of perfect homogeneity of a gas is incompatible with the fact that light is scattered in it. He realized that the blue color of the sky proves that the homogeneity of gases is only apparent. More precisely, gases appear homogeneous only when examined by coarse instruments, such as a barometer, scales or other instruments, which are simultaneously influenced by many billions of molecules. But the light beam senses incomparably smaller quantities of molecules, measured only in tens of thousands. And this is enough to establish beyond doubt that the density of the gas is continuously subject to small local variations. Therefore, a medium that is homogeneous from our "rough" point of view is in fact inhomogeneous. From the "point of view of light", it appears cloudy and therefore scatters light.
Random local changes in the properties of a substance resulting from the thermal motion of molecules are now called fluctuations. Having clarified the fluctuation origin of molecular scattering of light, Mandelstam paved the way for a new method of studying matter - the fluctuation, or statistical, method, which was later developed by Smolukhovsky, Lorentz, Einstein and himself into a new major department of physics - statistical physics.
The sky should be twinkling!
So, the secret of the blue sky was revealed. But the study of light scattering did not stop there. Paying attention to the almost imperceptible changes in the density of the air and explaining the color of the sky by fluctuation scattering of light, Mandelstam, with his sharpened instinct of a scientist, discovered a new, even more subtle feature of this process.
After all, air inhomogeneities are caused by random fluctuations in its density. The magnitude of these random inhomogeneities, the density of clumps, changes over time. Therefore, the scientist reasoned, the intensity should also change over time - the strength of the scattered light! After all, the denser the clumps of molecules, the more intense the light scattered on them. And since these clots appear and disappear chaotically, the sky, to put it simply, should flicker! The strength of its glow and its color should change all the time (but very weakly)! But has anyone ever noticed such a flicker? Of course not.
This effect is so subtle that you cannot see it with the naked eye.
None of the scientists also observed such a change in the glow of the sky. Mandelstam himself did not have the opportunity to verify the conclusions of his theory. The organization of the most complex experiments was hampered at first by the meager conditions of tsarist Russia, and then by the difficulties of the first years of the revolution, foreign intervention and civil war.
In 1925, Mandelstam became the head of the department at Moscow University. Here he met with an outstanding scientist and skilled experimenter Grigory Samuilovich Landsberg. And so, bound by deep friendship and common scientific interests, they together continued the storming of secrets hidden in the weak rays of scattered light.
The optical laboratories of the university in those years were still very poor in instruments. There was not a single device at the university that could detect the flickering of the sky or those small differences in the frequencies of incident and scattered light that the theory predicted are the result of this flicker.
However, this did not stop the researchers. They gave up the idea of imitating the sky in the laboratory. It would only complicate an already subtle experience. They decided to study not the scattering of white - complex light, but the scattering of rays of one, strictly defined frequency. If they know exactly the frequency of the incident light, it will be much easier to search for those frequencies close to it, which should arise during scattering. In addition, the theory suggested that observations are easier to carry out in solids, since in them the molecules are located much closer than in gases, and the scattering is the greater, the denser the substance.
A painstaking search for the most suitable materials began. Finally, the choice fell on quartz crystals. Simply because large transparent quartz crystals are more readily available than any other.
Preparatory experiments lasted two years, the most pure samples of crystals were selected, the technique was improved, signs were established by which it was possible to undoubtedly distinguish scattering on quartz molecules from scattering on random inclusions, crystal inhomogeneities and impurities.
Wit and hard work
Lacking powerful equipment for spectral analysis, the scientists chose an ingenious workaround, which was supposed to make it possible to use the available instruments.
The main difficulty in this work was that much stronger light was superimposed on the weak light caused by molecular scattering, scattered by small contaminants and other defects of those crystal samples that were obtained for experiments. The researchers decided to take advantage of the fact that the scattered light formed by crystal defects and reflections from different parts installation, exactly coincides in frequency with the incident light. They were only interested in light with a frequency changed in accordance with Mandelstam's theory. Thus, the task was to highlight the light of a changed frequency caused by molecular scattering against the background of this much brighter light.
In order for the scattered light to have a magnitude available for registration, the scientists decided to illuminate the quartz with the most powerful lighting device available to them: a mercury lamp.
So, the light scattered in a crystal should consist of two parts: from a weak light of a changed frequency, due to molecular scattering (the study of this part was the goal of scientists), and from a much stronger light of an unchanged frequency, caused by extraneous causes (this part was harmful, it made research difficult).
The idea of the method attracted by its simplicity: it is necessary to absorb light of a constant frequency and pass only light of a changed frequency into the spectral apparatus. But the frequency differences were only a few thousandths of a percent. No other laboratory in the world has had a filter capable of separating such close frequencies. However, a way out was found.
The scattered light was passed through a vessel with mercury vapor. As a result, all the “harmful” light “stuck” in the vessel, and the “useful” light passed without noticeable attenuation. In this case, the experimenters took advantage of one already known circumstance. The atom of matter, according to quantum physics, is capable of emitting light waves of only quite certain frequencies. At the same time, this atom is also capable of absorbing light. Moreover, only light waves of those frequencies that he himself can emit.
In a mercury lamp, light is emitted by mercury vapor, which is glowing under the influence of an electrical discharge that occurs inside the lamp. If this light is passed through a vessel that also contains mercury vapor, it will be almost completely absorbed. What the theory predicts will happen: the mercury atoms in the vessel will absorb the light emitted by the mercury atoms in the lamp.
Light from other sources, such as a neon lamp, will pass through the mercury vapor unharmed. Atoms of mercury will not even pay attention to it. The part of the light from the mercury lamp that is scattered in quartz with a change in wavelength will not be absorbed either.
It was this convenient circumstance that Mandelstam and Landsberg took advantage of.
Amazing discovery
In 1927, decisive experiments began. The scientists illuminated the quartz crystal with the light of a mercury lamp and processed the results. And ... they were surprised.
The results of the experiment were unexpected and unusual. Scientists have found not at all what they expected, not what was predicted by the theory. They discovered a completely new phenomenon. But which one? And isn't this a mistake? In the scattered light, not expected frequencies were found, but much higher and lower frequencies. A whole combination of frequencies appeared in the spectrum of the scattered light, which were not present in the light incident on the quartz. It was simply impossible to explain their appearance by optical inhomogeneities in quartz.
A thorough check began. The experiments were carried out flawlessly. They were conceived so cleverly, perfectly and ingeniously that one could not help but admire them.
- So beautifully and sometimes brilliantly, Leonid Isaakovich sometimes simply solved very difficult technical problems that involuntarily each of us had a question: "Why did it not occur to me before?" - says one of the employees.
Various control experiments have consistently confirmed that there is no error. In the photographs of the scattered light spectrum, weak and, nevertheless, quite obvious lines persisted, indicating the presence of "extra" frequencies in the scattered light.
For many months, scientists have been looking for an explanation for this phenomenon. Where did the “alien” frequencies come from in the scattered light ?!
And the day came when Mandelstam was struck by an amazing guess. It was an amazing discovery, the very one that is now considered one of the most important discoveries of the 20th century.
But both Mandelstam and Landsberg came to the unanimous decision that this discovery can be published only after a solid check, after an exhaustive penetration into the depth of the phenomenon. The final experiments began.
With the help of the sun
On February 16, Indian scientists C.N. Raman and K.S. Krishnan sent a telegram from Calcutta to this magazine with a short description of his discovery.
In those years, letters about a variety of discoveries flocked to the journal "Priroda" from all over the world. But not every message is destined to cause excitement among scientists. When the issue with a letter from Indian scientists came out of print, physicists were very excited. The very title of the note - "A new type of secondary radiation" - aroused interest. After all, optics is one of the oldest sciences; it was not often possible to discover something unknown in it in the 20th century.
One can imagine with what interest physicists all over the world were awaiting new letters from Calcutta.
Their interest was largely fueled by the very personality of one of the authors of the discovery, Raman. This is a man of curious fate and an outstanding biography, very similar to Einstein's. Einstein in his youth was a simple gymnasium teacher, and then an employee of the patent office. It was during this period that he completed the most significant of his works. Raman, a brilliant physicist, also after graduating from university was forced to serve in the department of finance for ten years, and only after that was invited to the department of the University of Calcutta. Raman soon became the recognized head of the Indian School of Physics.
Shortly before the events described, Raman and Krishnan were carried away by a curious task. Then the passions caused in 1923 by the discovery of the American physicist Compton, who, studying the passage of X-rays through matter, discovered that some of these rays, scattering away from the original direction, increased their wavelength, had not yet subsided. Translated into the language of optics, we can say that X-rays, colliding with the molecules of the substance, changed their "color".
This phenomenon was easily explained by laws quantum physics... Therefore, Compton's discovery was one of the decisive proofs of the correctness of the young quantum theory.
Something similar, but in optics, we decided to try. discovered by Indian scientists. They wanted to pass light through the substance and see how its rays will be scattered on the molecules of the substance and whether their wavelength will change at the same time.
As you can see, willingly or unwillingly, Indian scientists set themselves the same task as Soviet scientists. But their goals were different. In Calcutta, an optical analogy was sought for the Compton effect. In Moscow - an experimental confirmation of Mandelstam's prediction of frequency change in the scattering of light by fluctuating inhomogeneities.
Raman and Krishnan conceived a difficult experience, since the expected effect was to be extremely small. The experiment required a very bright light source. And then they decided to use the sun, collecting its rays with a telescope.
The diameter of his lens was eighteen centimeters. The researchers sent the collected light through a prism to the vessels, which contained liquids and gases, carefully cleaned of dust and other contaminants.
But it was hopeless to detect the expected small elongation of the scattered light using white sunlight, which contains almost all possible wavelengths. Therefore, scientists decided to use light filters. They put a blue-violet filter in front of the lens, and observed the scattered light through a yellow-green filter. They rightly decided that what the first filter missed would get stuck in the second. After all, the yellow-green filter absorbs the blue-violet rays transmitted by the first filter. And both, placed one after the other, must absorb all the incident light. If any rays get into the eye of the observer, then it will be possible to say with certainty that they were not in the incident light, but were born in the substance under study.
Columbus
Indeed, in the diffused light, Raman and Krishnan found rays passing through the second filter. They recorded extra frequencies. In principle, it could be optical effect Compton. That is, when scattered by the molecules of the substance in the vessels, the blue-violet light could change its color and become yellow-green. But this still had to be proved. There could be other reasons causing the yellow-green light to appear. For example, it could appear as a result of luminescence - a weak glow that often occurs in liquids and solids under the influence of light, heat, and other reasons. Obviously, there was one thing - this light was born again, it was not contained in the incident light.
The scientists repeated their experiment with six different liquids and two types of vapors. They made sure that neither luminescence nor other reasons play a role here.
The fact that the wavelength of visible light increased when it was scattered in matter seemed to Raman and Krishnan to be established. It seemed that the search for them was crowned with success. They found an optical analogy to the Compton effect.
But in order for the experiments to have a finished form and the conclusions were sufficiently convincing, one more part of the work had to be done. It was not enough to detect the change in wavelength. It was necessary to measure the magnitude of this change. The first helped to make a light filter. He was powerless to do the second. Here, the scientists needed a spectroscope - a device that can measure the wavelength of the light under study.
And the researchers began the second part, no less difficult and painstaking. But she also met their expectations. The results again confirmed the conclusions of the first part of the work. However, the wavelength turned out to be unexpectedly long. Much more than expected. Researchers were not embarrassed by this.
How not to remember Columbus here? He strove to find a sea route to India and, seeing the land, did not doubt that he had achieved his goal. Did he have reason to doubt his confidence in the sight of the Redskins and the unfamiliar nature of the New World?
Didn't Raman and Krishnan, seeking to discover the Compton effect in visible light, thought they had found it by examining the light that passed through their liquids and gases ?! Did they doubt when measurements showed an unexpectedly greater change in the wavelength of the scattered rays? What conclusion did they draw from their discovery?
According to Indian scientists, they found what they were looking for. On March 23, 1928, a telegram flew to London with an article entitled "Optical Analogy of the Compton Effect." Scientists wrote: "Thus, the optical analogy of the Compton effect is obvious, except that we are dealing with a change in wavelength much larger ..." Note: "much larger ..."
Dance of atoms
Raman and Krishnan's work was greeted with a standing ovation among scholars. Everyone rightly admired their experimental art. For this discovery, Raman was awarded the Nobel Prize in 1930.
A photograph of the spectrum was attached to the letter of Indian scientists, in which the lines took their places, depicting the frequency of the incident light and light scattered by the molecules of the substance. This photograph, according to Raman and Krishnan, illustrated their discovery more clearly.
When Mandelstam and Landsberg looked at this photo, they saw an almost exact copy of the photo they had taken! But when they got acquainted with her explanation, they immediately realized that Raman and Krishnan were wrong.
No, not the Compton effect was discovered by Indian scientists, but a completely different phenomenon, the same one that Soviet scientists have been studying for many years ...
While the excitement caused by the discovery of Indian scientists was growing, Mandelstam and Landsberg were finishing control experiments, summing up the last decisive results.
And on May 6, 1928, they sent an article to print. A photograph of the spectrum was attached to the article.
Briefly outlining the history of the issue, the researchers gave detailed interpretation the phenomenon discovered by them.
So what was this phenomenon that made many scientists suffer and break their heads?
Mandelstam's deep intuition and clear analytical mind immediately prompted the scientist that the detected changes in the frequency of the scattered light could not be caused by those intermolecular forces that equalize the random repetitions of air density. It became clear to the scientist that the reason undoubtedly lies within the molecules of the substance themselves, that the phenomenon is caused by intramolecular vibrations of the atoms that form the molecule.
Such fluctuations occur with a much higher frequency than those that accompany the formation and resorption of random inhomogeneities in the medium. It is these vibrations of atoms in molecules that affect the scattered light. Atoms, as it were, mark it, leave their traces on it, encrypt it with additional frequencies.
It was a most beautiful guess, a daring intrusion of human thought beyond the cordon of a small fortress of nature - a molecule. And this intelligence brought the most valuable information about its internal structure.
Hand in hand
So, when trying to detect a small change in the frequency of the scattered light caused by intermolecular forces, a larger change in frequency caused by intramolecular forces was found.
Thus, to explain the new phenomenon, which received the name "Raman scattering of light", it was enough to supplement the theory of molecular scattering, created by Mandelstam, with data on the influence of vibrations of atoms within molecules. The new phenomenon was discovered as a result of the development of Mandelstam's idea, formulated by him back in 1918.
Yes, not without reason, as Academician S.I. Vavilov, “Nature endowed Leonid Isaakovich with a completely unusual, perspicacious, subtle mind, which immediately noticed and understood the main thing that the majority passed by indifferently. This is how the fluctuation essence of light scattering was understood, and the idea of changing the spectrum during light scattering appeared, which became the basis for the discovery of Raman scattering. "
Subsequently, tremendous benefits were drawn from this discovery, it received valuable practical applications.
At the moment of its discovery, it seemed only the most valuable contribution to science.
What about Raman and Krishnan? How did they react to the discovery of Soviet scientists, and to their own too? Did they understand what they discovered?
The answer to these questions is contained in the following letter from Raman and Krishnan, which they sent to the press 9 days after the publication of the article by Soviet scientists. Yes, they understood - the phenomenon they observed was not the Compton effect. This is Raman scattering of light.
After the letters of Raman and Krishnan and the articles of Mandelstam and Landsberg were published, it became clear to scientists all over the world that one and the same phenomenon was independently and practically simultaneously made and studied in Moscow and Calcutta. But Moscow physicists studied it in quartz crystals, and Indian physicists - in liquids and gases.
And this parallelism, of course, was not accidental. She speaks about the urgency of the problem, its great scientific importance. It is not surprising that the results, close to the conclusions of Mandelstam and Raman at the end of April 1928, were independently obtained by the French scientists Rocard and Kaban. After a while, scientists remembered that back in 1923, the Czech physicist Smekal theoretically predicted the same phenomenon. Following the work of Smekal, the theoretical studies of Kramers, Heisenberg, Schrödinger appeared.
Apparently, only a lack of scientific information can explain the fact that scientists in many countries worked on solving the same problem, without even knowing about it.
Thirty seven years later
Raman scattering studies have not only opened a new chapter in the science of light. At the same time, they gave powerful weapons to technology. The industry has got a great way to study the properties of a substance.
After all, the frequencies of Raman light scattering are imprints that are superimposed on the light by the molecules of the medium that scatters the light. And in different substances, these prints are not the same. This is what gave Academician Mandelstam the right to call Raman scattering of light "the language of molecules." For those who can read the traces of molecules on the rays of light, determine the composition of the scattered light, molecules, using this language, will tell about the secrets of their structure.
On the negative of the photograph of the combination spectrum, there is nothing but lines of different blackness. But from this photo, the specialist will calculate the frequencies of intramolecular vibrations that appeared in the scattered light after it passed through the substance. The picture will tell about many hitherto unknown aspects of the inner life of molecules: about their structure, about the forces that bind atoms into molecules, about relative movements atoms. By learning to decipher Raman spectrograms, physicists learned to understand a kind of "light language" that molecules use to tell about themselves. So the new discovery made it possible to penetrate deeper into internal structure molecules.
Today physicists use Raman scattering to study the structure of liquids, crystals and vitreous substances. Chemists use this method to determine the structure of various compounds.
Methods for the study of matter, using the phenomenon of Raman scattering of light, were developed by the laboratory staff Institute of Physics named after P.N. Lebedev of the USSR Academy of Sciences, headed by Academician Landsberg.
These methods allow, in a factory laboratory, to quickly and accurately perform quantitative and qualitative analyzes of aviation gasolines, cracked products, refined petroleum products, and many other complex organic liquids. To do this, it is enough to illuminate the substance under study and determine the composition of the light scattered by it with a spectrograph. It seems very simple. But before this method turned out to be really convenient and fast, scientists had to work a lot on creating accurate, sensitive equipment. And that's why.
Of the total amount of light energy entering the substance under study, only an insignificant part - about one ten-billionth - falls on the scattered light. And the Raman scattering rarely accounts for even two or three percent of this value. This is probably why the Raman scattering itself remained unnoticed for a long time. And it is not surprising that obtaining the first Raman photographs required exposures lasting tens of hours.
Modern equipment, created in our country, makes it possible to obtain a combination spectrum of pure substances within a few minutes, and sometimes even seconds! Even for the analysis of complex mixtures, in which individual substances are included in an amount of several percent, an exposure of no more than an hour is usually sufficient.
Thirty-seven years have passed since the language of molecules, recorded on photographic plates, was discovered, deciphered and understood by Mandelstam and Landsberg, Raman and Krishnan. Since then, all over the world, persistent work has been carried out to compile a "dictionary" of the language of molecules, which optics call the catalog of Raman scattering frequencies. When such a catalog is compiled, the interpretation of spectrograms will be greatly facilitated and the Raman scattering of light will become even more fully at the service of science and industry.
Why the sky is blue? It is difficult to find an answer to such a simple question. Many scientists have racked their brains in search of an answer. The best solution to the problem was proposed about 100 years ago by an English physicist Lord John Rayleigh.
The sun emits dazzling white light. This means that the color of the sky should be the same, but it is still blue. What happens to white light in the earth's atmosphere?
White light is a mixture of colored rays. With the help of a prism, we can make a rainbow.
The prism splits the white beam into colored stripes:
■Red
■Orange
■ Yellow
■ Green
■ Blue
■ Blue
■ Purple
Combining together, these rays again form white light. It can be assumed that sunlight first splits into colored components. Then something happens, and only blue rays reach the surface of the Earth.
So why is the sky blue?
There are several possible explanations. The air surrounding the Earth is a mixture of gases: nitrogen, oxygen, argon and others. The atmosphere also contains water vapor and ice crystals. Dust and other small particles are suspended in the air. The upper atmosphere contains a layer of ozone. Could this be the reason? Some scientists believed that ozone and water molecules absorb red rays and let blue ones through. But it turned out that the atmosphere simply didn't have enough ozone and water to color the sky blue.
In 1869 an Englishman John Tyndall suggested that dust and other particles scatter light. Blue light is least scattered and passes through layers of such particles, reaching the surface of the Earth. In his laboratory, he created a model of the smog and illuminated it with a bright white beam. The smog turned deep blue. Tyndall decided that if the air were absolutely clear, then nothing would scatter the light, and we could admire the bright white sky. Lord Rayleigh also supported this idea, but not for long. In 1899, he published his explanation:
It is the air, not dust or smoke, that colors the sky blue.
Basic theory about blue sky
Part of the sun's rays passes between gas molecules without colliding with them, and without changes reaches the surface of the Earth. The other, large part, is absorbed by gas molecules. When photons are absorbed, the molecules are excited, that is, they are charged with energy, and then emit it in the form of photons. These secondary photons have different wavelengths and can be of any color, from red to violet. They scatter in all directions: to the Earth, and to the Sun, and to the sides. Lord Rayleigh suggested that the color of the emitted ray depends on the predominance of quanta of one color or another in the ray. When a gas molecule collides with photons of the sun's rays, there are eight blue quanta for one secondary red quantum.
What is the result? Intense blue light literally pours on us from all directions from the billions of molecules of gases in the atmosphere. This light is mixed with photons of other colors, so it does not have a pure blue tone.
Why then is the sunset red?
However, the sky is not always blue. The question naturally arises: if we see a blue sky throughout the day, why is the sunset red? The red color is least scattered by gas molecules. During sunset, the Sun approaches the horizon and the sunbeam is directed to the Earth's surface not vertically, as during the day, but at an angle.
Therefore, the path that he travels through the atmosphere is much greater than that he travels during the day, when the sun is high. Because of this, the blue-blue spectrum is absorbed in the thick layer of the atmosphere, before reaching the Earth. And longer light waves of the red-yellow spectrum reach the surface of the Earth, painting the sky and clouds in the typical red and yellow colors of the sunset.
Scientific explanation
Above, we gave the answer in relatively simple language. Below we will quote the rationale using scientific terms and formulas.
Excerpt from Wiki:
The sky looks blue because the air scatters short wavelength light more than long wavelength light. The intensity of Rayleigh scattering, caused by fluctuations in the number of air gas molecules in volumes comparable to the wavelengths of light, is proportional to 1 / λ 4, λ is the wavelength, that is, the violet portion of the visible spectrum is scattered approximately 16 times more intensely than red. Since blue light has a shorter wavelength at the end of the visible spectrum, it is more scattered in the atmosphere than red. Due to this, the area of the sky outside the direction of the Sun has a blue color (but not violet, since the solar spectrum is uneven and the intensity of the violet color in it is less, and also due to the lower sensitivity of the eye to violet and more to blue, which annoys not only those sensitive to blue color cones in the retina, but also sensitive to red and green rays).
During dusk and dawn, light travels tangentially to the earth's surface, so the path traversed by light in the atmosphere becomes much larger than during the day. Because of this, most of the blue and even green light is scattered from direct sunlight, so the direct light of the sun, as well as the clouds it illuminates and the sky near the horizon, are colored red.
Probably, with a different composition of the atmosphere, for example, on other planets, the color of the sky, including at sunset, may be different. For example, the sky on Mars is reddish pink.
Scattering and absorption are the main causes of the attenuation of light intensity in the atmosphere. Scattering varies as a function of the ratio of the diameter of the scattering particle to the wavelength of light. When this ratio is less than 1/10, Rayleigh scattering occurs, in which the scattering coefficient is proportional to 1 / λ 4. At larger values of the ratio of the size of scattering particles to the wavelength, the scattering law changes according to the Gustave Mee equation; when this ratio is greater than 10, the laws of geometric optics are applicable with sufficient accuracy for practice.
When the wind throws a white fluffy transparent cape over the beautiful blue sky, people begin to look up more and more often. If at the same time it also puts on a large gray fur coat with silver threads of rain, then those around it are hiding from it under umbrellas. If the outfit is dark purple, then everyone sits at home and wants to see the sunny blue sky.
And only when such a long-awaited sunny blue sky appears, which wears a dazzling blue dress decorated with golden sun rays, people rejoice - and smiling, leave their homes in anticipation of good weather.
The question of why the sky is blue has worried human minds since time immemorial. Greek legends have found their answer. They claimed that the purest rhinestone gives it this shade.
In the days of Leonardo da Vinci and Goethe, they were also looking for an answer to the question of why the sky is blue. They believed that the blue color of the sky is obtained by mixing light with darkness. But later this theory was refuted as untenable, since it turned out that by combining these colors, you can get only the tones of the gray spectrum, but not the color one.
After a while, the answer to the question why the sky is blue was tried to explain in the 18th century by Marriott, Bouguer and Euler. They believed that this is the natural color of the particles that make up the air. This theory was popular even at the beginning of the next century, especially when it was found that liquid oxygen is blue, and liquid ozone is blue.
Saussure was the first to give a more or less sensible idea, who suggested that if the air were completely clean, without impurities, the sky would turn out to be black. But since the atmosphere contains various elements (for example, steam or water droplets), they, reflecting color, give the sky the desired shade.
After that, scientists began to get closer and closer to the truth. Arago discovered polarization, one of the characteristics of scattered light that bounces off the firmament. In this discovery, the scientist was definitely helped by physics. Later, other researchers began to look for the answer. At the same time, the question of why the sky is blue was so interesting for scientists that a huge number of different experiments were carried out to clarify it, which led to the idea that the main reason for the appearance of blue is that the rays of our Sun are simply scattered in the atmosphere.
Explanation
The first to create a mathematically sound answer to molecular light scattering was the British researcher Rayleigh. He put forward the assumption that light is scattered not because of the impurities that the atmosphere possesses, but because of the air molecules themselves. His theory was developed - and these are the conclusions that the scientists came to.
The sun's rays make their way to the Earth through its atmosphere (thick layer of air), the so-called air shell of the planet. The dark sky is completely filled with air, which, despite the fact that it is completely transparent, is not a void, but consists of gas molecules - nitrogen (78%) and oxygen (21%), as well as water droplets, steam, ice crystals and small pieces of solid material (e.g. dust particles, soot, ash, ocean salt, etc.).
Some rays manage to pass freely between gas molecules, completely bypassing them, and therefore reach the surface of our planet without changes, but most of the rays collide with gas molecules that come into an excited state, receive energy and release multi-colored rays in different directions, completely coloring the sky. as a result, we see a sunny blue sky.
White light itself is made up of all the colors of the rainbow, which can often be seen when it is decomposed into its component parts. It so happened that air molecules scatter the most blue and violet colors, since they are the most short part spectrum, since they have the shortest wavelength.
When mixed in an atmosphere of blue and purple with small amounts of red, yellow and green, the sky begins to "glow" blue.
Since the atmosphere of our planet is not homogeneous, but rather different (it is denser near the surface of the Earth than above), it has a different structure and properties, we can observe overflows of blue color. Before sunset or sunrise, when the length of the sun's rays increases significantly, the blue and violet colors are scattered in the atmosphere and do not reach the surface of our planet at all. The yellow-red waves are successfully reaching, which we observe in the firmament during this period of time.
At night, when the sun's rays hitting a certain side of the planet have no opportunity, the atmosphere there becomes transparent, and we see the "black" space. This is how it is seen by astronauts above the atmosphere. It is worth noting that the astronauts were lucky, because when they are over 15 km above the earth's surface, during the day they can simultaneously observe the Sun and the stars.
The color of the sky on other planets
Since the color of the sky is largely influenced by the atmosphere, it is not surprising that different planets it is of different colors. Interestingly, Saturn's atmosphere is the same color as on our planet.
The sky of Uranus is very beautiful aquamarine. Its atmosphere is composed primarily of helium and hydrogen. It also contains methane, which completely absorbs red and scatters green and blue colors. Of blue color Neptune's skies: This planet's atmosphere does not have as much helium and hydrogen as ours, but a lot of methane, which neutralizes the red light.
The atmosphere on the Moon, the satellite of the Earth, as well as on Mercury and Pluto is completely absent, therefore, light rays are not reflected, so the firmament is black here, and the stars are easily distinguishable. The blue and green colors of the sun's rays are completely absorbed by the atmosphere of Venus, and when the Sun is near the horizon, the skies are yellow.