In the article you can find out a simple explanation of the blue (with shades) color of the sky. After all, the question is actually very interesting, especially for children. Let's find a simple explanation for this phenomenon, although this is not as easy as it seems.
The human eye is only able to see three colors, not as the popular belief is that the eye can see many colors. They are red, green and blue.
Introduction: why is the sky blue?
Photographic film is built exactly on the above principle. There are three surfaces in the frame, each one perceives only its own light, changing color in accordance with the absorption of rays. When the light of an electric lamp passes through it, creating an image on the screen, we see millions of shades, due to their mixing in different proportions. Technique copies nature. After all, the human eye works precisely according to this principle. It contains such biological elements that only react to their own color.
And when these colors are mixed in the human brain, we observe the color that reflects the object. For example, mixing blue and yellow produces green. An interesting fact is that we see yellow as paler than blue or green. This is a color deception of the human eye. The black and white photograph clearly shows that the yellow is not pale at all.
We only see the color that is reflected from the surface. For example, Europeans have white skin, while Africans have almost black skin. This only suggests that in some skin coloration is capable of reflecting all colors, which happens when all three primary colors are mixed, while in others it is only absorbing. After all, we see only reflected rays. Ideally, of course, absolutely white and absolutely black skin does not exist. But I wrote it so that it was clearer.
Answer: why is the sky blue?
“But what does the sky have to do with it? - a reader, already wise by experience, will say now, - is the sky capable of reflecting rays? Agree. It allows them to pass, but the air surrounding the Earth, which extends for a thousand kilometers above the surface, does not allow all the rays to pass through. He partially delays red and green, and blue skips. Therefore, looking at the sky, we see it blue, blue, and in bad weather purple and even lead. The human eye, unlike various objects, practically does not reflect light, but only absorbs with its cones and rods, which are sensitive to a certain color. And since the blue spectrum of the rays predominates, we see it.
The sky looks blue because the air scatters short wavelength light more than long wavelength light.
But this does not mean that the sky cannot be red, crimson, scarlet or pink. At least his plots. If you watch it at sunrise or sunset, then you will be amazed at the riot of bloody colors. But you will not see a green, yellow sky. Why is this happening? At sunrise or sunset, the sun does not penetrate the atmosphere from above, but at a very small angle, so we see a bloody dawn or a crimson sunset.
In simple terms, the sky is blue, because when light decomposes, purple is the most scattered, and red is the least.
Light through a prism
As you know, white light consists of seven primary colors that change as the wavelength of light decreases: red, orange, yellow, green, blue, blue and violet. And, for example, astronauts in orbit see a dazzling white sun against a black sky. This is as it should be: the constituent parts of white light reach them in an airless space without distortion, while they reach the Earth through the "filter" of the atmosphere.
If we consider in more detail, then you need to understand what is diffuse sky radiation - solar radiation reaching the earth's surface after it has been scattered by molecules or particulate matter in the atmosphere. Of all the solar radiation scattered in the atmosphere, about two-thirds eventually reaches the Earth as diffuse radiation (if the Sun is high above the horizon, at least 25% of the incident radiation is scattered).
The main mechanisms of light scattering in the atmosphere (Rayleigh scattering, Mie scattering) are elastic, that is, a change in the direction of radiation occurs, without changing the wavelength.
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 \u200b\u200bthe 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 color of 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 large values \u200b\u200bof the ratio of the size of scattering particles to the wavelength, the scattering law changes according to the Gustave Mie equation; when this ratio is greater than 10, the laws of geometric optics are applicable with sufficient accuracy for practice.
Why is the sky blue - it is very difficult to find an answer to such a simple question. Many scientists racked their brains in search of an answer. The best solution to the problem was proposed about 100 years ago by the English physicist Lord John Rayleigh.
But let's start over. 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?
A little about color
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, light blue, blue and violet. 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.
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Hypotheses put forward at different times
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, Englishman John Tyndall suggested that dust and other particles scattered 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 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.
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The relationship between color and wavelength - why the sky is blue
Part of the sun's rays passes between gas molecules without colliding with them, and without changes reaches the Earth's surface. 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.
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Features of the color spectrum
Before reaching the surface of the earth, where people can contemplate it, the sunlight must pass through the entire air envelope of the planet. Light has a wide spectrum, in which the main colors, shades of the rainbow, stand out. Of this spectrum, red has the longest wavelength of light, while violet has the shortest. At sunset, the sun's disk rapidly turns red and rushes closer and closer to the horizon.
In this case, the light has to overcome an increasing thickness of air, and some of the waves are lost in this case. First purple disappears, then blue, cyan. The longest red waves continue to penetrate to the Earth's surface to the last, and therefore the solar disk and the halo around it have reddish tints until the last moments.
Why is the sky blue during the day?
Long light waves can penetrate deep into the atmosphere for the reason that they are almost not absorbed, not scattered by aerosols and suspensions that are constantly circulating in the planet's atmosphere. When the luminary is closer to the zenith, a different situation develops, which provides the blue sky. Blue has shorter wavelengths than red and is more strongly absorbed. But its scattering ability is 4 times higher than that of red.
