Question

A beam of light of wavelength $\lambda$ is incident on a mirror at an angle $\theta$. Find the change in momentum of the photon after reflection.

• A. $2\frac{h}{\lambda }sin\theta$
• B. $\frac{h}{\lambda }cos\theta$
• C. $2\frac{h}{\lambda }cos\theta$
• D. none of these

Answer 1

Answer: (C)

$2\frac{h}{\lambda }cos\theta$

After reflection the horizontal component of velocity remains same and normal component reverses its direction

$\Delta P=2P\cos \theta$

For photon $P=\frac{h}{\lambda }$

$\Delta P=2\frac{h}{\lambda }\cos \theta$