Why is the sky blue?

It is due to the effect of the scattering of light radiation. It is called Rayleigh scattering.

The­re is a physical phenomenon called Rayleigh scattering that causes light to scatter when it p­asses through particles that have a diameter one-tenth that of the wavelength (color) of the light. Sunlight is made up of all different colors of light, but because of the elements in the atmosphere the color blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.