Why is sky's colour blue?

The sky appears blue because of a phenomenon called Rayleigh scattering, which is the scattering of light by small particles or molecules in the atmosphere.

The shorter, blue wavelengths of light are scattered more than the longer, red wavelengths, resulting in the blue color we see in the sky.

Sunlight enters Earth's atmosphere and is made up of a spectrum of colors. The smaller molecules of gases such as nitrogen and oxygen in the atmosphere scatter the light.

The blue light is scattered more than the other colors because it travels as shorter, smaller waves. 

This scattered blue light reaches our eyes from all directions, giving the sky its blue appearance.