Why does the sky appear blue on a clear day?

Hint: Explaining the concept of scattering of light needs to be done here. Sunlight which is reaching to Earth’s surface is scattered through many gases and particles in the air.

Complete answer:
Phenomenon of scattering: It is a phenomenon in which light gets deviated from its straight path and gets spread in all different directions when striking an obstacle or dust or molecules present in the air.
Now when a straight white light travelling in air passes through any obstacle lets say prism gets scattered in all directions.
Similarly when a blue light passes through air it gets scattered in all directions by tiny molecules of air. Blue is scattered more than others because it travels as shorter smaller waves. This is the reason why we see a blue sky on a clear day.

Note: It is important to have a clear idea about scattering of light. One should have a proper knowledge about visible colour and their frequency range. A white light when passes through a prism gets scattered into seven visible colours known as VIBGYOR. Visible light has a frequency range of $4 \times {10^{^{14}}}$ to $8 \times {10^{18}}$ cycles per second.