A laser beam of wavelength 500 nm and power 5 mW strikes normally on a perfectly reflecting surface of area 1 mm^2 of a body. It rebounds back from…

No. of photons ejected per second \(\displaystyle =\frac{P}{E}\) \[=\frac{P}{\left(\dfrac{h c}{\lambda}\right)} \] Momentum of photon, \(\displaystyle p=\frac{h}{\lambda}\) Force exerted by laser beam \[\begin{aligned} F & =2 \times \frac{P}{\left(\dfrac{h c}{\lambda}\right)} \times \frac{h}{\lambda} \\ & =\frac{2 P}{c} \\ & =\frac{2 \times 5 \times 10^{-3}}{3 \times 10^{8}} \\ F & =3.33 \times 10^{-11} \mathrm{~N} \end{aligned} \]