The clear sky is blue, not because the atmosphere itself is blue, nor because it contains blue substances, but because atmospheric molecules and tiny particles suspended in the atmosphere scatter sunlight. Due to the inhomogeneity of the medium. The phenomenon that light deviates from the original propagation direction and scatters sideways is called the scattering of light by medium. The scattering of fine particles follows Rayleigh's law: the intensity of scattered light is inversely proportional to the fourth power of wavelength. When sunlight passes through the atmosphere, violet, blue and cyan light with short wavelength are most easily scattered; However, the red, orange and yellow light with longer wavelength scatter weakly, and the sky appears blue due to this comprehensive effect.
This is because the sun's rays are scattered by atmospheric molecules and particles suspended in the atmosphere after being emitted into the atmosphere. According to the determination of scientists, the wavelengths of blue light and purple light are relatively short, which is equivalent to "wavelet"; The wavelengths of orange light and red light are relatively long, which is equivalent to "big waves". When encountering obstacles in the air, blue light and purple light are scattered everywhere because they can't cross those obstacles, filling the whole sky. That's how the sky turns blue. Tindor scattering
There are many tiny dust, water droplets, ice crystals and other substances in the air. When sunlight passes through the air, blue, purple, indigo and other short-wavelength colored rays are easily blocked by particles suspended in the air, which makes the light scatter in all directions and the sky appears blue. In the Chinese world, most authoritative educational science websites, large and small, still adopt the above-mentioned "standard answer".
This explanation of "sky blue" is basically the level of1mid-9th century. It was initiated by British physicist john tyndall (1820- 1893). 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 of 19, Ding's explanation of Tianlan was questioned.
rayleigh scattering
In the1880s, John Rayleigh (1842- 19 19) noticed that it was unnecessary to resort to airborne particles, such as dust, water droplets and ice crystals. 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 concluding article on the origin of blue sky (J. Rayleigh, Phil. Margo. XLVII, 375, 1899), and said at the beginning: "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.
Einstein's theory
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. " The physics of the origin of "sky blue" was not initiated by Einstein, but the most complete theory was laid by Einstein. So the physics of "Sky Blue" was completed in 19 10.
Rayleigh and Einstein's "sky blue" theory 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.
The first physical formula is quoted by Mr. Gao Kun in his first paper (C. Kao, Proc. IEE,113,7, 1966 20 10) laid the foundation for "optical fiber communication" by Einstein's "sky blue" Rayleigh scattering formula (that is, glass is a solidified liquid. Even if the ideal glass has no bubbles and no defects, there are still indelible' impurities' in the glass, that is, the indelible fluctuations of the glass itself. The signal (light wave) propagating in optical fiber will be scattered by the fluctuation of glass. The "sky blue" mechanism is the physical main reason for the loss of optical fiber communication signals. This cannot be eliminated by optical fiber manufacturing technology. Only "not too blue" light can be selected to reduce its influence.