A red laser of wavelength 662 nm shines on a black bead, which experiences a force of 1 N as a result. Let R be the rate (number of photons per second) at which photons are emitted by the laser. Assuming idealized situations (full absorption of light by the bead, environment is a vacuum, etc.), calculate R.

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anonymous · Answer

1uN

anonymous · Answer

Each photon carries a momentum of $$h/\lambda $$ Force is defined as change in momentum divided the  time over which the momentum is changed. ie$$F=\Delta p/\Delta t$$
the change in momentum in this case is the momentum transferred by the absorption of N photons.  ie.$$\Delta p=N(h/\lambda )  $$F is given so  solve for $$N/ \Delta t$$ the photons absorbed per second.