The principle that explains the differential scattering of shorter wavelength radiation and accounts for the earth's blue sky is Rayleigh Scattering.
The blue color of the sky is caused by sunlight dispersion off the molecules of the atmosphere. This type of scattering is more effective at short wavelengths. Thus, the light reflected and scattered down the earth at a large angle in reference to the direction of the rays of the sun is predominantly the blue end of the spectrum.
