White weather

On the earth, the color of the sun is red at sunset and sunrise, and basically white at noon. The reason is related to the scattering of sunlight in the atmosphere, because the thickness of the atmosphere through which sunlight passes is different at sunrise, sunset and noon.

As we know, sunlight is composed of seven colors, which can be separated by a prism, namely, red, orange, yellow, green, blue, indigo and purple.

The earth has a thick atmosphere, and sunlight will pass through it before reaching the ground. Because the atmosphere can absorb and scatter sunlight, the sunlight reaching the ground will change.

In particular, the scattering effect of the atmosphere on solar light is very obvious. The main components of the atmosphere are various gas molecules, the scale of which is in the nanometer scale and the scale is in the micrometer scale. There are two main scattering mechanisms of sunlight penetrating the atmosphere.

(1) Rayleigh scattering

The wavelength of visible light is longer than gas molecules. When sunlight passes between gas molecules, it will cause Rayleigh scattering of sunlight. The intensity of Rayleigh scattering is inversely proportional to the fourth power of wavelength, so the shorter the wavelength, the easier it is to scatter, and the scattering ability of blue light is stronger than that of red light.

(2) Michaelis scattering

The wavelength of visible light is equivalent to the size of dust. At this time, the scattering caused by dust is called Michaelis scattering, and the intensity of Michaelis scattering is inversely proportional to the second power of wavelength.

For the earth's atmosphere, in the case of good weather (except foggy weather, haze weather, etc.). ), the molecular density of gas in the atmosphere is much higher than that of dust, so Rayleigh scattering is more likely to occur.

At low latitudes, at noon, the height angle of the sun is the largest, and the thickness of the atmosphere through which sunlight needs to pass is the shortest. At this time, the sunlight is basically a mixture of seven colors, so it is white; The blue-violet light scattering is the most obvious. The sky is blue because human eyes are sensitive to blue light.

At sunset and sunrise, the sun's altitude angle is small, and sunlight needs to pass through the farther atmosphere to reach the ground. At this time, the most scattered blue-violet light is a spent force, while the most scattered red light still exists, so the sun is red.