Question

A parallel beam of monochromatic light of wavelength 663 nm is incident on a totally reflecting plane mirror. The angle of incident is ${60}^o$ and the number of photons striking the mirror per second is $1.0\times {10}^{19}$. If the force exerted by light beam on the mirror is $Y\times {10}^{-8}N$. Find Y.

Answer 1

momentum $p=\frac{h}{\lambda }$ = ${10}^{-27}Kgm/s$

Force exerted by 1 photon = change in momentum = $2\ast pCosi=p$

Force exerted by ${10}^{19}$ photons =${10}^{-27}\ast {10}^{19}={10}^{-8}N$

So force extered in ${10}^{-8}N$ is 1