A parallel beam of monochromatic light is incident normally on a perfectly reflecting surface. The incident radiation has the power as $2.1387\times {10}^{-27}\text{W}$. The force exerted by the light beam on the surface by each photon is $(h=6.63\times {10}^{-34}\text{Js})$

The correct option is B $1.4\times {10}^{-19}\text{N}$
For a perfectly reflecting surface, force exerted by a radiation is,

$F=\frac{2p}{c}=\frac{2\times 2.1387\times {10}^{-27}}{3\times {10}^8}$

$=\frac{4.2774\times {10}^{-27}}{3\times {10}^8}$

$=1.42\times {10}^{-19}\text{N}$

Hence, option $\left(\text{B}\right)$ is correct.

Why this question? Key concept: For a perfectly reflecting surface, force exerted by a radiation is, $F=\frac{2p}{c}$