When we lie on the grass, the breeze blows, the blue sky comes into view, and the body and mind feel relaxed, have you ever wondered why the sky is blue? Why not other colors, such as pink, green?
It is this natural phenomenon that we have become accustomed to, but the hidden physical laws are not simple, and it is not simple, because the correct explanation of "why the sky is blue" was not perfected by scientists until the end of the 19th century and the beginning of the 20th century, when more than two centuries had passed since Newton's creation of the classical mechanical system. However, from the perspective of popular science articles, we only need to understand these laws qualitatively, so that it is not complicated.
We don't know who was the first in history to explain why the sky is blue (the explanation doesn't need to be correct), but we can think about it for ourselves.
First of all, no matter what color the sky is, the premise is that the photons in the air enter our eyes, so that we can see the state of the sky, and where do these photons come from?
To know that the atmosphere is impossible to emit light on its own (it should be said that in the absence of foreign objects, it is impossible to continue to emit visible light), and the photons here are naturally "sent" by the sun, but we all know that sunlight is composite light, because Newton used a prism to find that sunlight can be decomposed into a variety of colors of light, so is the blue of the sky also because there is something similar to a triangular prism in the sky, which decomposes the light?
But if you think about it, this is not true, the sunlight through the prism will decompose into several colors of light, that is because of refraction, if so, the sky we see should not be just a single blue, but colorful. If it is not the cause of refraction, is it because the blue light is "blocked" on the way through the atmosphere, causing them to be unable to reach the ground, and can only continue to "wander" in the atmosphere?
In general, this is an optical phenomenon involving the passage of light through a medium, so we can start with a similar Tyndall effect.
About this effect, I believe that many readers and friends have contact when learning middle school chemistry, the teacher told us that the Tyndal effect can be used to distinguish between the solution and colloid, you only need to use a beam of light to align the liquid to be distinguished, if there is a bright path inside, it means that this is a colloidal solution.
The Tindal effect was discovered by british scientist John Tindal in 1869, and the microscopic explanation behind it is because there are a large number of small particles with a diameter of no more than 100 nanometers in the colloidal solution, and the diameter of these particles is smaller than the visible wavelength range, so when light passes through, the phenomenon of scattering occurs (if the diameter of the particles is too large, much larger than the wavelength of light, there will be a reflection phenomenon)
Therefore, people at that time used the Tyndall effect to explain the color of the sky, believing that it was due to the presence of a large number of small particles such as dust, small water droplets, ice crystals, etc. in the atmosphere, and sunlight would inevitably encounter these particles when passing through the atmosphere, scattering the blue light with shorter wavelengths in the sunlight into the entire sky, so the entire sky would appear blue.
This explanation does not seem to be a problem, but in fact, in the Tyndall effect, the correlation between the intensity of the scattered light and the wavelength of the incident light is not very strong, although it can be said in theory in the past, but in the present, it is not the correct answer, the key problem lies in the "dust, water droplets and other small particles in the atmosphere" this point.
We know that although it is "natural" that the air contains small particles similar to dust and water droplets, the concentration of these small particles is a variable, and the concentration directly affects the degree of scattering, and the difference in the degree of scattering directly leads to differences in the color of the sky.
But in fact, there seems to be no difference between the sky on the steppe and the sky on the desert, which is obviously inconsistent with the explanation of the Tyndall effect. If it can be summed up, it is the Tyndall effect that explains "why the sky is blue", and its conclusion should be that the difference in color of the sky will be obvious due to the different environments in different regions.
But in fact, this does not exist, so why does the sky appear blue?
Now that we have been able to rule out the effects of small particles of impurities in the atmosphere (at least not the main cause), what else is there? When you think about it, it seems that there is only the atmosphere itself, is it because of the various gas molecules in the atmosphere?
Fortunately, our idea came together with the famous physicist John William Strath (that is, Baron Rayleigh, so posterity directly called him Rayleigh, which is also reflected in the physicist Kelvin, whose real name is actually William Thomson)
After careful study, Rayleigh found that scattering occurs not only in impurity particles, but also in individual atoms or molecules. This is called Rayleigh scattering, in general, when the diameter of the particle is much smaller than the wavelength of the incident light (no more than one-tenth of the wavelength), the intensity of the scattered light is proportional to the frequency of the incident light in a quadratic proportional relationship (that is, it is inversely proportional to the wavelength of the quaternion), so the shorter the wavelength of the light, the stronger the scattering, if the sunlight is taken as an example, then the short-wave blue-violet light located in the visible light range is most likely to be scattered.
Then for a very pure atmospheric environment, even if there is no suspended impurity particles such as various dust and ice crystals inside, it will also cause the blue-violet light in the sunlight to be scattered away due to the scattering of atmospheric molecules, thus diffusing into the entire atmosphere, so Rayleigh scattering is the reason why the sky is blue.
And the sunset at sunset is the best test of this theory.
Considering that the earth is a sphere, because the atmosphere that wraps the earth can be seen as a spherical shell, generally around noon every day, the position of the sun reaches the highest point of the day, when the sunlight penetrates from the atmosphere to the ground, the thickness of the atmosphere experienced is the shortest in the day; and at sunset (or sunrise), the thickness of the atmosphere experienced by the sunlight is the longest in the day, which is more intuitively seen from the following figure
The longer the distance traveled, the more blue-violet light is scattered, so that only the reddish light reaches the ground, which is why the sun shows the color of salted duck egg yolk at sunset, but it is worth noting that this red only appears near the sun, while most of the rest of the sky still shows blue or darker colors (after all, the sun is about to set, and it is about to enter the night)
This concludes this article.
Thank you for reading!
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This article is original by Mr. Sai Science Popularization, welcome to pay attention, take you along with long knowledge!