Question

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.

Answer 1

1uN

Answer 2

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.