question_answer 4)

Monochromatic light of wavelength 632-8 nm 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), and (c) How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon?  

Answer:

Given, Power, (a) Also, (b) Number of photons emitted per second,  (c) Mass of hydrogen atom  mass of proton  using