Blue colour of sky is due to the phenomenon of scattering.
What is meant by scattering?

Answer

a) Scattering is the change in direction brought out by the irregular and partial reflectiön of light when it hits the particlés of medium.
b) When rays of light pass through a colloidal fluid or suspension, the tiny particles get illuminated due to scattering. Because of this, the path of light is made visible. This phenomenon is Tyndal Effect.
c) During sunset the sun is near the horizon and the light rays have to pass through á larger distances to reach our éyes. Consequently the lower wavelengths in the blue region are almost. completely scattered away by the air molecules. The higher wavelengths of the red region are least scttered'and reach our eyes. Hence the horizon appear to have red colours.