1 Answer

"It is due to Rayleigh scattering, but the sky is not always blue"

Rayleigh scattering is the name for what happens to light when it passes through the atmosphere of the Earth. The longer red, orange, and yellow wavelengths pass straight through without much interference, but the shorter wavelengths in the blue range can be absorbed by the molecules in the atmosphere, and then re-radiated in a different direction. This is why the whole sky looks bright, instead of the sun looking like a flashlight in the night. The blue wavelengths of light in effect, strike the atmosphere from the same direction, and then are scattered in all directions, giving the sky its color. You get blue from everywhere, but the other colors only from one direction (the sun).

Don’t ever look directly at the sun, but if you were to do so, you would see that the sky changes color from blue to white, not because of the intensity, but because you would be receiving all of the color wavelengths, not a preponderance of the blue.

During sunsets, the sky turns yellow, orange, and red. Being on the edge of the lighted part of the Earth, less of the scattered blue wavelengths are scattered in your direction. Only the yellow, orange, and red wavelengths reach you, giving you those colors.