Why is the sky blue?

The reason why the sky is blue is because: "there are indelible impurities in the air, that is, the fluctuation of the air itself." The scattering of solar light by density fluctuations forms a blue sky. "

Sky is blue (short for Sky is Blue) originated from physics and was completed at 19 10. The theory of "sky blue" laid by Rayleigh and Einstein is universally applicable. It can be used to explain the "blue sky" phenomenon in pure air, and it can also be used to explain the "blue sky" phenomenon in pure water, pure glass and other liquids or solids.

"Sky Blue" physics seems to be very popular. Every junior high school student who has seen "100,000 Why" can tell its "standard answer": "There are many tiny dust, water droplets, ice crystals and other substances in the air. When sunlight passes through the air, colors with shorter wavelengths such as blue, purple and indigo are easily blocked by particles suspended in the air, thus scattering light in all directions and making the sky appear blue. " In the Chinese world, most authoritative educational science websites, regardless of size, still use the above-mentioned "standard answers", almost word for word. This explanation of "sky blue" is basically the level in the middle of the nineteenth century. It was initiated by British physicist Tindal. It is often called Tindor scattering model. Indeed, "blue light with short wavelength is easily blocked by particles suspended in the air, ... and scattered in all directions." But it is not the real reason for "sky blue". If the sky blue is mainly caused by the scattering of water droplets, ice crystals and other particles, then the color and depth of the sky should change with the change of air humidity. Because when the humidity changes, the number of water droplets and ice crystals in the air will change obviously. There is a great difference in humidity between wet areas and desert areas, but the sky is the same blue. Tindal scattering model can't explain it. By the end of19th century, Ding's explanation of sky blue was questioned.

In the1880s, Rayleigh noticed that there was no need to resort to dust, water droplets, ice crystals and other particles in the air. Oxygen, nitrogen and other molecules in the air scatter sunlight, and blue light is easily scattered. Therefore, the scattering of air molecules can be the main cause of "sky blue". However, the scattering of each molecule does not mean that the air as a whole will be blue. If the pure air is extremely uniform, there will be no "sky blue" with many molecules. Just like a very flat mirror, it only refracts or reflects and rarely scatters. In a uniform environment, the scattering of different molecules cancels each other out. Just like in an environment with strong collective discipline (such as prison), everyone's independent and sloppy behavior is completely compressed. And "sky blue" depends on the independence of molecules and no interference or less interference. For this reason, Rayleigh assumes that air is not a "prison" for molecules. On the contrary, molecules such as oxygen and nitrogen walk randomly and distribute randomly. The Rayleigh quantitative results calculated by this model are very consistent with the properties of sky blue. 1899, Rayleigh wrote a summary article "On the Origin of the Blue Sky", which began by saying: "Even without foreign particles, we will still have a blue sky". "Foreign particles" are needed for Tindal scattering. Since then, Tindor's sky blue theory has been abandoned. Rayleigh scattering has become the mainstream of "sky blue" theory. Although Rayleigh's sky blue theory is very successful, Rayleigh's hypothesis about the random distribution of molecules is also well founded.

But Rayleigh essentially assumes that air is the so-called ideal gas, which is a small but not negligible weakness. Because air is not an ideal gas. 19 10 years, Einstein finally solved this problem. Einstein proved that even the purest air will fluctuate with the newly developed statistical thermodynamic theory of entropy (a measure of chaos). The density fluctuation of air itself can also be scattered, and blue light is easily scattered. The scattering of density fluctuation, neither too much nor too little, can just produce the blue sky we see. If air is an ideal gas, Einstein's result is the same as Rayleigh's.

So, to put it simply, the cause of the blue sky is: "There are indelible' impurities' in the air, which is the fluctuation of the air itself. The scattering of solar light by density fluctuations forms a blue sky. "