Question

EM Waves having a uniform and constant, intensity I, are incident normally on a (perfectly black) surface of area A, for a time t. The average force (F), exerted on the surface by these em waves, depends on the time (t) in the manner shown in graph

(1) W

(2) X

(3) Y

(4) Z

Answer 1

(1) W

When EM Waves, of intensity I, are incident normally, on a surface of area A for a time t, the total energy absorbed by the (black) surface is

U = I x A x t

Therefore, the total momentum, delivered to the surface, is

The average force thus does not depend on the time, t, for which the em waves are incident on. Hence the graph, between F and t, has the form shown in graph W.