Why the sky is blue, illustration. Sunlight entering the Earth's atmosphere is scattered in different proportions depending on its wavelength. Known as Rayleigh scattering, this occurs to a greater extent for blue light than for other colours in the visible light spectrum (such as red). This results in indirect blue light coming from all parts of the sky (diffuse sky radiation) and this is why the sky appears blue in the daytime from the surface of the Earth.