Question

The radiation force due to source of power P on a perfectly reflecting surface will be:

• A. $\frac{P}{3c}$
• B. $\frac{2P}{c}$
• C. $\frac{P}{2c}$
• D. $\frac{P}{c}$

Answer 1

Answer: (B)

$\frac{2P}{c}$

Let the energy be E,

So momentum $p=\frac{E}{c}$

now rate of change of momentum $dp/dt=\frac{dE}{cdt}=\frac{P}{c}$ ,because $dE/dt=Power$

After reflection from mirror it will travel in opposite direction.

Now Force is equal to change in momentum $F=\frac{P}{c}-\frac{-P}{c}=\frac{2P}{c}$