Why the sky is blue in day?

                                            

Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the forward and backward directions than it is in the lateral direction.Scattering is significant for light at all visible wavelengths, but it is stronger at the shorter (bluer) end of the visible spectrum; meaning that that the scattered light is more blue than its source, the sun. The remaining sunlight, having lost some of its short wavelength components, appears slightly less blue. Although violet, not blue, is the color with the shortest visible wavelength; the spectrum of light emission from the sun is not constant at all wavelengths, and additionally is partially absorbed by the high atmosphere, so there is less violet in the light. Human eyes are also less sensitive to violet than blue.