Question: Why is the sky blue?

Marcus Gallagher-Jones answered on 19 Mar 2012:

I know this one. This is actually due to the light being scattered by air molecules in the atmosphere. The longer wavelengths of light, reds and yellows, will pass straight over these air molecules however the shorter wavelengths, blues and purples will be absorbed. They will then be re-emitted in a different direction by a process known as rayleigh scattering. This happens all over the place so that even though the origin of light is only coming from one direction, the sun, blue light will be scattered everywhere giving a blue sky. The reason the horizon appears paler than the main bulk of the sky is because the lue light must travel further. This increases the chance of it being scattered again and so less light will reach your eyes giving a paler colour.