When white light from the sun enters the earth's atmosphere, the scattering of different colours (i.e., wavelength) takes place due to interaction with large number of very small molecules in the earth's atmosphere. The wavelength of blue colour is much smaller than that of red colour. Therefore, intensity of blue light scattered is much more than that of the red colour (∵I∝1∕λ4). Thus the blue colour becomes the major colour when the sky is clear. Due to this fact, the sky appears blue.