choose the right option:
A plane electromagnetic wave of energy U is reflected from the surface. Then the momentum transferred by em
wave to the surface
(a) 0 (b) U/c (c) 2U/c (d) U/2c

Dear student,
If the surface is a perfect reflector, such as a mirror, and the incidence is normal, then the momentum transported to the surface in a time t is twice that given above,
U=mc2 , and momentum(p) is mc  so here p= U/c
Let K be the momentum transferred to the surface 
after striking the electromagnetic wave will travel in the opposite direction to the initial direction.
from conservation of momentum 
initial momentum = final momentum 
U/c + 0 = K - U/c 
K = 2U/c
i.e., the momentum transferred to the surface by the incoming light is p = U/c, and that transferred by the reflected light also is p = U/c. Therefore, K = 2U/c is the momentum transferred by em wave to the surface. 
Regards

 

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