Question

A parallel beam of monochromatic light of wavelength 663 nm is incident on a totally reflecting plane mirror. The angle of incidence is 60° and the number of photons striking the mirror per second is 1.0 x 10¹⁹ . Calculate the force exerted by the light beam on the mirror.

Answer 1

λ  = 663 x 10^–9 m,  θ= 60°, n = 1 x 10¹⁹, λ = h/p

=> P = p/λ = 10^–27

Force exerted on the wall = n(mv cosθ –(–mv cosθ)) = 2n mv cos θ.

= 2 x 1 x 10¹⁹ x 10^–27 x ½ = 1 x 10^–8 N.