Why is the sky purple at night?

The color of the sky is actually related to aerosols in the atmosphere. Theoretically any color can appear. For example, when a volcano erupts, a large number of S ions enter the atmosphere, and the color of the sky is not blue. If it is purple, it may be photochemical smog that day. The atmosphere was originally transparent, but the scattering of solar light made it become. At night, the sun disappears, and you see the color of the universe. As for the sky, it is purple, mainly a mixture of various colors of light emitted by stars in the universe.

The color of the sky is actually related to aerosols in the atmosphere. Theoretically any color can appear. For example, when a volcano erupts, a large number of S ions enter the atmosphere, and the color of the sky is not blue. If it is purple, it may be photochemical smog that day. The reason why the sky is blue is that sunlight is made up of seven kinds of light. Of the seven kinds of light, cyan, blue and purple have shorter wavelengths, and light waves are most easily scattered by air molecules and dust in the air, so when sunlight passes through the atmosphere, most of the three kinds of light are 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.

The color of the sky (that is, the color of the atmosphere) is actually the composite color around the blue in the spectrum, which we call "blue". If there is no atmosphere, the sun we see is a very dazzling fireball in the dark space background, and astronauts in the space station can also see this scene, because sunlight loses most of the blue region of the spectrum through the atmosphere, and the sun we often see through the atmosphere is yellow. When blue light and yellow light are mixed together, it is white light (sunlight that has not passed through the atmosphere).

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.