Under what circumstances will a purple sky appear?

It is composed of light of different colors.

The atmosphere at the top of the sky is thin, so the blue of the zenith is scattered in the sky. When the blue of the zenith overlaps with the red of the horizon, the sky appears purple.

Of the seven kinds of light, cyan, blue and purple have shorter wavelengths, and the light with shorter wavelengths is most easily scattered by air molecules and dust in the air, so when sunlight passes through the atmosphere, most of cyan, blue and purple will be scattered. Purple is almost invisible in the atmosphere, because most purple light is absorbed when scattered, and the human eye is not sensitive to purple.

Extended data

Sunlight is white light, which consists of red, orange, yellow, green, blue and purple light. Red, orange, yellow and other colors have long wavelengths, which can pass through small particles and molecules in the sky, while blue and purple have short wavelengths, which are difficult to pass through, so they are scattered in the sky.

The sky in rainy days is gray, because thick clouds mainly reflect sunlight, so the light is weaker than sunny days, and the sky looks darker than sunny days. When sunlight passes through the thick clouds again, there is more water and dust in the clouds. After reflection, sunlight mainly reflects the colors of dust and small water droplets or small ice crystals, so the rainy sky looks gray.

Baidu Encyclopedia-Sky

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