A beam of light of wavelength `lambda` is totally reflected at normal incidence by a plane mirror . The intensity of the light is such that photons hit the mirror . The intensity of the light is such that photons hit the mirror at a rote `n` . Given that the Planck constant is `h` , the force exerted on the mirror by this beam is

Options

A. `nh lambda`
B. `nh//lambda`
C. `2nh//lambda`
D. `2nlambda//h`

Answer

D

Explanation

Let `p` be the momentum of a photon arriving at the mirror , we then have `p = (h)/(lambda)`
The change in momentum for a photon
`= p - (- p) = 2 p = ( 2h)/( lambda)`
Force = rate of change of momentum of the photons
` = n ((2h)/(lambda))` , where `n` is the rate at which the photons hitting the mirror .