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If the thickness of the atmosphere changes, is the sky still blue?

2025-01-18 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >

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Under the optical action of scattering, light encounters atmospheric molecules and countless suspended particles, changing the energy and direction and spreading in all directions. As a result, the clear sky is blue, the sunset is orange-red, the waves are rolling white spray, and every fluctuation in the micro world may cause thousands of changes in the colors of nature. So, what are the cases of scattering? when we look at the sky at different atmospheric heights, is the sky the same color? And if the thickness of our atmosphere changes, will the sky still be blue? Come and explore.

Atmospheric scattering is the phenomenon that when a light beam encounters atmospheric molecules or aerosol particles, the light propagates around with particles as the center. There are three common types of scattering: Rayleigh scattering, meter scattering and Raman scattering. The combination of these three phenomena can explain many optical phenomena in life.

Figure 1 schematic diagram of the scattering phenomenon, the sun shines on the cloud droplets, the light is disturbed and shoots in all directions. First of all, I would like to introduce to you the Rayleigh scattering that we hear most often. When the particle size is much smaller than the incident light wavelength (less than 1/10 of the wavelength), the scattering is Rayleigh scattering, and the scattered light intensity is different in all directions. The scattering is the strongest in the incident direction and the weakest in the vertical incident direction (figure 2, left). For Rayleigh scattering, the scattering intensity is inversely proportional to the fourth power of the incident light wavelength, as shown on the right of figure 2, that is, the longer the incident light wavelength, the weaker the scattering (less likely to be scattered by particles) and the stronger the penetration ability; the shorter the wavelength, the stronger the scattering (the easier it is to be scattered by particles), and the weaker the penetration ability is.

Figure 2 schematic diagram of Rayleigh scattering (top) and variation of Rayleigh scattering intensity with wavelength (bottom). In a clear sky, there are less dust and water vapor, mainly atmospheric molecules. Rayleigh scattering often occurs because the wavelength of sunlight (400-800 nm) is much larger than the diameter of atmospheric molecules (about 1 nm). At noon, sunlight (light of various frequencies and colors) is directed into the atmosphere. After a certain distance, most of the blue light with shorter wavelengths is scattered in all directions by atmospheric molecules, while the red light with longer wavelengths is scattered less. So the sky we see is blue. Because the blue-purple light in solar radiation is scattered away, when we look at the sun directly, the sun presents complementary colors, that is, yellow.

Fig. 3 schematic diagram of the incidence of sunlight into the atmosphere early in the morning or evening, the sunlight oblique into the atmosphere, the distance of the light path is longer than that of noon, and the blue light is almost scattered in the upper atmosphere. As the light continues to spread to the ground, the atmosphere mainly scatters red light, so the sunset and sunset are red (figure 4).

Fig. 4 in the evening sky, the upper atmosphere mainly scatters blue light, while the lower atmosphere mainly scatters red light. When we stand on the ground, we will see red morning glow and sunset. Here, you may wonder that purple light has a shorter wavelength than blue light and should be scattered more easily, but why is the sky not purple? There are three reasons: first, according to the solar radiation spectrum, the radiation intensity of each wavelength is different, and the radiation intensity of purple light is lower than that of blue light (figure 5), so the energy of purple light reaching our eyes is very weak; second, the ozone layer also strongly absorbs ultraviolet rays (including purple light); third, human eyes are not sensitive to purple light.

Fig. 5 Rayleigh scattering of solar radiation spectrum occurs when the particle is much smaller than the wavelength of incident light. If the particle size is close to or larger than the wavelength of the incident light, the scattered light intensity is asymmetric in all directions, and most of the incident light is scattered along the forward direction (figure 6). This phenomenon is called meter scattering. The intensity of meter scattering (yellow area of figure 6) is much stronger than that of Rayleigh scattering (blue area of figure 6), and the change of scattering intensity with wavelength is not as intense as Rayleigh scattering. When the wavelength increases gradually, the scattering intensity tends to a certain value in the form of vibration.

Figure 6: comparison of Rayleigh scattering and Rayleigh scattering the clouds in the sky are formed by the accumulation of a large number of small water droplets and small ice crystals, most of which are 0.01-0.1mm in diameter and longer than the wavelength of sunlight. As a result, when light passes through the clouds, meter scattering occurs, and the light scattering intensity of different wavelengths is not very different. When they are mixed together, the scattered light is as white as the sun, and most of the clouds we see are white. When precipitation is about to occur, the atmospheric water vapor content is higher, the small water droplets condensed in the clouds are getting larger and larger, the refraction and absorption effect of sunlight is enhanced, and the clouds become darker. This is the dark cloud we see. By the same token, there are more dust particles in the dirty atmosphere, and most of them are particles with larger diameters. At this time, the light will also be scattered by meters. The white scattered light fills the air, and the sky looks gray.

Both meter scattering and Rayleigh scattering are elastic scattering, that is to say, the frequency and energy of the beam do not change in the process of propagation, and red light is emitted at the far end, and what is seen by the human eye is red light. If the frequency of the beam changes when it propagates in a transparent medium, for example, in figure 7, the excited light is green, but the colors we see in the middle of the beam are yellow and red, this phenomenon is called Raman scattering, which is one of the inelastic scattering. The intensity of Raman scattering is very weak, which is about 1/1000 of Rayleigh scattering.

Figure 7 Raman scattering effect (Picture Source: little Red Book @ shuihua) finally answers our first two questions:

(1) if we look at the sky at different atmospheric heights, is the sky the same color?

The atmospheric density decreases sharply with height, that is, there are fewer gas molecules in the upper atmosphere and more in the lower atmosphere. Therefore, from the lower level to the upper level, the scattering effect of atmospheric molecules decreases accordingly, and the color of the sky changes from azure to cyan (about 8km), dark cyan (about 11km), dark purple (about 13km), dark purple (about 21km), and then up, the air is very thin, the scattering effect of atmospheric molecules is extremely weak, and the sky is annihilated by darkness.

(2) if the thickness of our atmosphere changes, will the sky still be blue?

In fact, we most often see two colors (blue and red) in the sky, with an implicit condition that the thickness of the atmosphere does not change. If the atmosphere becomes thicker and the scattering, reflection and absorption of sunlight by atmospheric molecules are enhanced, the color of the sky in our eyes will become lighter, darker, or almost white. If it becomes infinitely thick, there will be no light transmission. If the atmosphere becomes thinner and more light is scattered than blue light, then we can see that the sky will be darker and appear dark blue.

reference

1. Atmospheric Physics, Sheng Peixuan et al., Peking University Press

two。 Encyclopedia: Rayleigh scattering, meter scattering

3. Technical Art personal Notes (8)-- Rayleigh scattering and Michaelis scattering, CSDN

This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), by Yang Liu

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