Why the sky is blue - it is very difficult to find the answer to such a simple question. The best solution to the problem was proposed about 100 years ago by the English physicist Lord John Rayleigh.

But let's start again. So the color of the sky should be the same, but it is still blue. What happens to white light in the earth's atmosphere?

The color of the sun

The real color of the sun's rays is white. White light is a mixture of colored rays. Using a prism, we can make a rainbow. Prism splits a white ray into colored stripes: red, orange, yellow, green, blue, blue and violet. Joining together, these rays again form white light. It can be assumed that sunlight first splits into colored components. Then something happens, and only the blue rays reach the surface of the Earth.

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. Water vapor and ice crystals are also present in the atmosphere. Dust and other small particles are suspended in the air. In the upper atmosphere is a layer of ozone. Could this be the reason?

Some scientists believed that ozone and water molecules absorb red rays and transmit blue ones. But it turned out that there was simply not enough ozone and water in the atmosphere to turn the sky blue.

In 1869, the Englishman John Tyndall suggested that dust and other particles scatter light.Blue light is scattered to the least extent and passes through layers of such particles, reaching the surface of the Earth. In his laboratory, he created a smog model and lit it with a bright white beam. The smog turned a deep blue.

Tyndall decided that if the air were absolutely clean, 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 air, not dust or smoke, that colors the sky blue.

The relationship of color and wavelength

Part of the sun's rays passes between the gas molecules, without colliding with them, and without changes reaches the surface of the Earth. The other, most, is absorbed by gas molecules. When photons are absorbed, molecules are excited, that is, charged with energy, and then emit it in the form of photons again. These secondary photons have different wavelengths and can be of any color - from red to purple.

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 sunlight, one blue secondary quantum has eight blue quanta.

What is the result? Intense blue light literally pours on us from all sides from billions of molecules of atmospheric gases. Photons of other colors are mixed with this light, so it does not have a pure blue tone.

Why is the sky blue - the answer

Before reaching the surface of the earth, where people can contemplate it, sunlight must pass through the entire air shell of the planet. Light has a wide spectrum, in which the basic colors and shades of the rainbow still stand out. From this spectrum, red has the longest light wave, while violet has the shortest. At sunset, the solar disk blushes rapidly and rushes closer and closer to the horizon.

In this case, light has to overcome an ever-increasing thickness of air, and part of the waves are lost. First violet disappears, then blue, cyan. The longest red waves continue to penetrate to the surface of the Earth to the last, and therefore the solar disk and the halo around it until the last moments have reddish hues.

What changes in the evening?

Closer to sunset, the Sun rushes to the horizon, the lower it falls, the faster the evening approaches. At such moments, the atmospheric layer that separates the original sunlight from the earth's surface begins to increase sharply due to the angle of inclination. At some point, the thickening layer ceases to transmit other light waves besides the red ones, and at this moment the sky turns this color. Blue is no longer present; it is absorbed as it passes through the atmosphere.

Interesting fact: at sunset, the sun and sky pass through a whole gamut of shades - as one or another of them ceases to pass through the atmosphere. The same can be observed at the time of sunrise, the causes of both phenomena are the same.

What happens when the sun rises?

At sunrise, the sun's rays go through the same process, but in reverse order. That is, first the first rays break through the atmosphere at a strong angle, only the red spectrum reaches the surface. Therefore, the sunrise initially shimmers in red. Then, as the sunrise and angle changes, waves of other colors begin to pass - the sky turns orange, and then it becomes habitually blue. A half-day deep blue of the sky is observed, and then, in the evening, it begins to again turn to the purplish. On one side of the sky, far from the sun, a blue-black hue is observed, but the closer to the setting sun, the more red hues can be seen near the horizon until the sun disappears completely.

Such color phenomena are observed everywhere. The sun becomes red, as are areas of the sky close to it, both at the equator and at the poles. This phenomenon can be seen all over the planet. Sometimes sunsets or sunrises have more saturated red tones, this is due to the state of the atmosphere, the presence of aerosols or suspensions in it. In other cases, the color is not so pronounced, more moderate. There are folk signs that allow you to determine the weather the next day by the shades of sunset - people have learned how to analyze colors and predict the state of the atmosphere from them with greater or lesser accuracy.

Thus, the red color of sunset is due to the fact that at a large angle through the atmosphere, only the red colors of the solar spectrum break through, having the longest wavelength. The red color of the sunrises is associated with the same factor.The rest of the day the sky is blue, as this shade is able to drown out the other spectrum, having the greatest ability to scatter.