Compared with the wavelength of visible light (about 400 nm to 700 nm), particles such as dust and water droplets in the air can be called behemoths. Take PM2.5, which has been wheezing recently, as an example, which refers to the pollution caused by particles suspended in the air with a particle size of less than or equal to 2.5 microns. 2.5 microns equals 2500 nanometers, which is far greater than the wavelength of visible light in sunlight, so when sunlight meets these particles, they will reflect in different directions. However, this reflection has the same effect on light with different wavelengths (or colors). In other words, particles like dust still reflect all colors of white light. Don't you believe it? When the air pollution index such as PM2.5 hits the rankings again, just look up and see what color the sky is-it should be said that there is no sky, only a piece of white. ...
So why is the sky blue? In fact, there are a large number of particles in the air whose scale is smaller than the wavelength of visible light, that is, the "diameter" of many gas molecules in the air, such as oxygen and nitrogen molecules, is about 0.3 nm. When these gas molecules are encountered, they will absorb some photons. After a while, the molecule will release another photon. The emitted photons are the same color as the absorbed photons, but the direction has changed. Although photons of all colors are absorbed, photons with higher frequency (blue) are more easily absorbed than photons with lower frequency (red). This process is called Rayleigh scattering, which is named after the British physicist Sir John Rayleigh, who first described this process in the 1970s.
Then, blue light is more likely to Rayleigh scatter with air molecules, and how can it produce blue sky? Let's make a simple hypothesis. If there were no air, what color would the sky be? Although most of us have never been to space, we can see from the documentary of Apollo landing on the moon that even if the sun shines in the sky, the sky is still black. The reason is as follows:
Because there is Rayleigh scattering in the air, the situation is completely different. On the way of sunlight propagating in the atmosphere, blue light is more prone to Rayleigh scattering and deflects in a direction different from the original direction of sunlight. Therefore, even if we don't look directly at the sun, but look in other directions of the sky, there are always photons scattered by air molecules (more blue light) that enter our eyes, so we see the blue sky. As shown in the figure below.
In fact, President Fang Lao wrote a long article on this topic, which is much more detailed and accurate than what I have here. If you are interested, it is recommended to read: "Why is the sky blue?
"Why is the sky blue?" The correct physical interpretation was completed in 19 10, and it has been one hundred years since then. An important application of "Sky Blue" physics is optical fiber communication, which was a project that Mr. Gao Kun won the Nobel Prize in Physics last year.
"Sky Blue" physics seems to be very popular. All junior high school students who have seen "100,000 Why" can say its "standard answer":
"There will be many tiny dust, water droplets, ice crystals and other substances in the air. When sunlight passes through the air, colored light with short wavelength such as blue, purple and indigo is 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 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 of19th century, Ding's explanation of sky blue was questioned.
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 summary article on the origin of sky blue [1], beginning with:
Even without foreign particles, we will still have 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 blue sky is caused by:
"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. Of course, this theory also has some inapplicability. Many years ago, I heard someone sing "I love the blue sky of my motherland" to the "blue sky". Don't mistake it for "I love the independence of my motherland but wander around".
In his first paper [3], Mr. Gao Kun cited the first physical formula that laid the foundation for "optical fiber communication" as Einstein's "sky blue" Rayleigh scattering formula (Einstein-Small Rukovski formula). 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.
Many authoritative education and science (Chinese) websites are reporting that Mr. Gao is the most influential Chinese in the world. Mr. Gao's influence is indeed worldwide. Interestingly, these websites themselves do not seem to be among the "affected". For example, the "sky blue" explanation quoted at the beginning of this article has not been "affected" at all. For teenagers, those "standard explanations" are not poisoned milk powder, but also milk powder that has expired for one hundred years.