What is the essential condition for the formation of beats?

What is the necessary condition for the formation of stationary waves ?

State the essential condition for the addition of vectors.

The essential condition for demodulation is

The essential condition for demodulation is

what is a rainbow ? What is the essential condition for observing it ?

With the help of an example, explain the process of hydrogenation. Mention the essential conditions for the reaction and state the change in physical property with the formation of the product.