A parellel beam of monochromatic light of wavelength 500 nm is incident normally on a perfectly absorbing surface. The power through any cross section of the beam is 10 W. Find
(a) the number of photons absorbed per second by the surface and
(b) the force exerted by the light beam on the surface.

Answer

A, B, C

Explanation

(a) Number of photon incident per second
= number of photon absorbed per second
`= (Power)/(Energy of one photon)`
`N = (P)/(hc//lambda) = (Plambda)/(hc)`
(b) Force = Rate of change of momentum =
(Number of photons absorbed per second)xx
(momentum of one photon)
`= ((Plambda)/(hc)) ((h)/(lambda)) = (P)/(c )) (:. P = power).`