blue sky

blue sky

Blue sky, the earth's atmosphere, is usually blue. In the middle of the 19th century, British physicist Dindal thought that the blue light with higher frequency was easily blocked by particles suspended in the air and scattered to all directions, which is still widely accepted in China's basic education. However, there are obvious loopholes in this statement. Later in the 1880s, Rayleigh found that oxygen and nitrogen molecules in the air scatter sunlight, while blue light (high frequency light) is easy to be scattered. The scattering of air molecules can be the main cause of "sky blue". In 1910, Einstein scientifically explained the reason of blue sky, that is, the scattering of sunlight by the density fluctuation of air itself formed blue sky.

Color principle

Traditional view

The blue sky is actually the atmosphere of the earth. The theory of John Tyndall (1820-1893), a British physicist in the middle of the 19th century, is mainly used to explain the reason for the emergence of "blue sky" in the field of basic education and popular science in China, although it was later proved not entirely correct. a clear sky is blue, not because the atmosphere itself is blue, nor does it contain blue substances, but because atmospheric molecules and tiny particles suspended in the atmosphere scatter sunlight. Because of the inhomogeneity of the medium. The phenomenon that light deviates from the original propagation direction and scatters to the side is called scattering of light by medium. The scattering of fine particles follows Rayleigh's Law: the intensity of scattered light is proportional to the fourth power of frequency. When the sunlight passes through the atmosphere, the green, blue and purple light with higher frequency is most likely to be scattered, while the red, orange and yellow light with lower frequency are less scattered. Due to this combined effect, the sky appears blue. this is due to the scattering of atmospheric molecules and particles suspended in the atmosphere after the sun rays pass through the atmosphere. According to the determination of scientists, if the frequency of green light, blue light and purple light is higher, the wavelength is shorter, which is equivalent to "small short leg"; if the frequency of red light, orange light and yellow light is lower, the wavelength is longer, which is equivalent to "big long leg". When encountering obstacles in the air, the blue light is "scattered" everywhere because of its "small and fast pace" (high frequency and short wave length), covering the whole sky. This is how the sky is "scattered" into blue.

The development of opinion

Tyndall scattering

there are many tiny dust, water droplets, ice crystals and other substances in the air. When the sunlight passes through the air, the green, blue, purple and other colors with higher frequency are easily blocked by the particles suspended in the air, so that the light is scattered to all directions and the sky appears blue. In the Chinese language world, most authoritative educational and scientific websites still use the above "standard answers". This "sky blue" explanation is basically the level of the mid-19th century. John Tyndall (1820-1823), a British physicist, pioneered it. It is often called the dingdal scattering model. Indeed, "blue light with higher frequency is easily blocked by particles suspended in the air The scattering direction is square. But it's 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 drop ice crystals in the air will change significantly. Humidity in humid and desert areas varies greatly, but the sky is the same blue. The dingdar a scattering model can't explain it. By the end of the 19th century, Ding's explanation of sky blue had been questioned.

Rayleigh scattering

in 1880s, John Rayleigh (1842-1919) 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 scattered sunlight, and blue light was 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, no matter how many molecules there are, there will be no "sky blue". Like a very flat mirror, only refraction or reflection, and very little scattering. In a uniform environment, the scattering of different molecules counteracts each other. Just like in an environment with strong collective discipline (such as prison), everyone's independent and loose behavior is completely compressed. And "sky blue" depends on the independence of molecules and mutual non-interference, or less interference. For this reason, Rayleigh assumes that air is not the "prison" of molecules. On the contrary, molecules like oxygen and nitrogen walk randomly and randomly. Rayleigh's quantitative results from this model are in good agreement with the properties of sky blue. In 1899, Rayleigh wrote a summary article "on the origin of blue sky" (j.rayleigh, phil.mag.xlvii, 3751899), which said at the beginning: "even if there are no foreign particles, we will still have blue sky". "Foreign particles" refers to the particles needed for dingdar scattering. Since then, dingdar'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 assumption of random molecular distribution is well founded. However, Rayleigh actually has to assume that air is the so-called ideal gas, which is a small but not negligible weakness. Because air is not an ideal gas.

Einstein Theory

In 1910, Einstein finally solved the problem. Einstein used the newly developed statistical thermodynamics theory of entropy (chaotic measurement) to prove that even the purest air has fluctuations. The density fluctuation of air itself can also be scattered, and blue light is easy to be scattered. The scattering of density fluctuations, no more, no less, 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 reason why the sky is blue is: "there are some unavoidable" impurities "in the air, that is, the fluctuation of the air itself. Density fluctuations and other scattering of sunlight, forming a blue sky The origin of "sky blue" physics was not initiated by Einstein, but the most complete theory was established by Einstein. So the "sky blue" physics was completed in 1910. Rayleigh and Einstein's "sky blue" theory is universally applicable. It can be used to explain the phenomenon of "blue sky" in pure air, and it can also be used to explain the phenomenon of "blue sky" in pure water, pure glass and other liquid or solid. The first physical formula cited by Mr. Kao in his first paper (C. Kao, proc. IEE, 113, No. 71966, 2010) laying the foundation for "optical fiber communication" is Einstein's "sky blue" Rayleigh scattering formula (Einstein Smoluchowski formula). Glass is a solidified liquid. Even if the most ideal glass has no bubbles or defects, there are still "impurities" in the glass, that is, the fluctuation of the glass itself. The signal (light wave) propagating in the optical fiber will be scattered by the fluctuation of the glass. The "sky blue" mechanism is a major physical cause of the loss of optical fiber communication signals. It cannot be eliminated by optical fiber manufacturing technology. You can only choose "not too blue" light to reduce its influence.

Air quality terminology

"Blue sky" is also a popular way to evaluate the air quality. China's environmental protection departments need to monitor many pollutants in the air, such as inhalable particulate matter, sulfur dioxide, nitrogen dioxide, etc. The real-time monitoring data are collected and calculated to get the air pollution index (API) of the day, so as to judge the level of air quality. the air pollution index adopted in China can be divided into five grades. API value less than or equal to 50 indicates that the air quality is excellent; API value greater than 50 and less than or equal to 100 indicates that the air quality is good. If the air quality pollution index on that day is below 100, it is called "blue sky". That is to say, although the sky doesn't look blue, and even it's raining, the air quality is good, which can still be regarded as "blue sky". therefore, the "blue sky" based on the air pollution index is not the blue sky in the sense of sense. It is reported that two citizens have been persistently taking a photo every day to record the number of blue sky. The photo shows that the number is 180 days. In the same period, the number of "blue sky" officially released by Beijing was 285 days, a difference of more than 100 days.

Chinese PinYin : Lan Tian