Question

A plane electromagnetic wave is incident on a plane surface of area $A$, normally and is perfectly reflected. If energy $E$ strikes the surface in time $t$ then force exerted on the surface is ( $c=$ speed of light)

• A. $\frac{2E}{Atc}$
• B. $\frac{E}{2c}$
• C. $\frac{2E}{ct}$
• D. Zero

Answer 1

Answer: (C)

Incident momentum, $p=\frac{E}{c}$
For perfectly reflecting surface with normal incidence
$\Delta p=2 p=\frac{2 E }{c}$ ;
$F =\frac{\Delta p}{\Delta t}=\frac{2 E }{c t}$