A continuous He–Ne laser (wavelength 633 nm) operates at a steady output power of 5.0 mW
PHYSICS · Multi-part · A Level · Physics · Particles and radiation · Particles and radiation
Problem
A continuous He–Ne laser (wavelength 633 nm) operates at a steady output power of 5.0 mW. The beam is expanded so that it uniformly illuminates a perfectly absorbing circular spot of radius 1.0 cm on a thin, floating foil.
Universal constants that may be useful: Planck constant h=6.63×10−34Js, speed of light in vacuum c=3.00×108ms−1.
(a) Determine the magnitude of the linear momentum carried by a single photon in the laser beam.
(b) Calculate the number of photons that strike the foil each second.
(c) Determine the average force exerted on the foil while it operates as a perfectly absorbing surface.
(d) If the foil were coated so that it became a perfectly reflecting mirror, calculate the new average force and state by what factor the force changes compared with part (c).