Question

(a)Why are infra-red waves often called heat waves ? Explain.
(b) What do you understand by the statement, "Electromagnetic waves transport momentum" ?

Answer 1

(a) Infra-red waves are also known as heat waves because they raise the temperature of the object on which they fall. They also affect the photographic plate and are readily absorbed by most materials. (b) Electromagnetic waves transport momentum. This means that when an e.m. wave travels through space with energy U and speed c, then it transports linear momentum p = $\frac{U}{c}$ . If a surface absorbs the waves completely, then momentum 'p' is delivered to the surface. However, if the surface reflects the wave, then momentum delivered by both incident and reflected wave adds on to give a momentum of '2p'.