Question

Light in certain cases may be considered as a stream of particles called photons. Each photon has a linear momentum $\frac{h}{\lambda }.$ where $h$ is the Planck's constant and $\lambda$ is the wavelength of the light.A beam of light of wavelength $\lambda$ is incident on a plane mirror at an angle of incidence $\theta$. Calculate the change in linear momentum of photon as the beam is reflected by the mirror.

Answer 1

Initial momentum of photon =$\frac{h}{\lambda }\cos \theta \hat{i}-\frac{h}{\lambda }\sin \theta \hat{j}$

Final momentum of photon=$\frac{-h}{\lambda }\cos \theta \hat{i}-\frac{h}{\lambda }\sin \theta \hat{j}$

$\therefore$ Change in linear momentum = Final - Initial

$=\frac{-2h}{\lambda }\cos \theta \hat{i}$