that's one I know!

sunlight is scattered by the atmosphere predominantly by Rayleigh scattering, which is light scattering by electric dipoles. it's much stronger at higher frequencies (that is, the higher the frequency, the more light is scattered). It's actually quite a strong dependence, goes like freq^4.

Since all the molecules in the atmosphere scatter the blue end of the spectrum much more strongly than the red, the sky is blue. It's bluest at 90 degrees between you and the sun because dipole scattering is strongest there.