Why sky in sunny days are blue?
The reason why the sky appears blue when it's sunny is because of Rayleigh scattering. This effect occurs when light particles (i.e., photons) collide with molecules and atoms of air, which scatter some of them towards shorter wavelengths than they would have otherwise gone to. As a result, these scattered rays are emitted from our eyes in all directions but preferentially forward direction, thus creating an illusion that we perceive the whole sky as being one homogeneous color.