Why the colour of clear sky is blue?

Why the colour of clear sky is blue?

The atmosphere of earth is full of particles or molecules like N2, O2, O3, H2, H2O, dust particles etc. These are so small that we cannot see them with naked eyes. Even with powerful microscope of 100X or higher available in school labs, air particles are not visible (except dust particles). When sunlight encounters these particles there is a change in the direction of sun rays which lead to a phenomenon known as scattering of light.The sunlight consists of different frequencies from 430 – 770 THz (or wavelengths 390 -700 nanomoetres). The colour of the scattered light depends upon the size of the encountered particles. The process of selective scattering is known as Rayleigh scattering.

The size of the air particles has size comparable to the wavelength of visible light at the blue end. These particles are more suitable in scattering light of blue and violet colours as compared to red colour. This is the reason why sky appears blue in colour.

Due to pollution, large size particles are introduced in the atmosphere. These particles are efficient in scattering light of longer wavelengths also. This causes the pale blue or grey colour of the sky in a polluted atmosphere. 

Watch the video shown below on youtube. This video shows how a laser light is scattered by water and talcom powder. The scattering of laser by water or talcom powder makes its path visible (known as Tyndall's  effect) and this property can be used to study relection or refraction.