The clear sky appears blue because of:

 

Solution:

Concept: If white light passes through a glass prism, it splits into its spectrum of colors (in order violet, indigo, blue, green, yellow, orange, and red), and this process of splitting white light into its constituent colors is known as dispersion.

Because of light dispersion, the clear sky appears blue. A clear, cloudless daytime sky is blue because molecules in the air scatter more blue than red light from the sun. 

At sunset, we see red and orange colors because blue light has been scattered out and away from our line of sight.