Monochromatic light of wavelength `632.8nm` is produced by a helium-neon laser. The power emitted is `9.42 mW`. (a) Find the energy and momentum of each photon in the light beam. (b) How many photons per second, on the average, arrive at a target irradiated by this beam? (Assume the beam to have uniform cross-section which is less than the target area). (c) How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon. `h=6.663xx10^(-34)Js,1 a.m.u. =1.66xx10^(-27)kg`.

Explanation

(a) `3.14xx10^(-19)J, 1.05xx10^(-27)kgm//s`
(b) `3xx10^(16)"photons/s"`
(c) `0.63m//s`