Answer to Question #120752 in Physics for peace

Question

A pulsed laser has an average output power of 12W. Each pulse consists of light at
wavelength 400nm and has 16ms duration. Compute the number of photons that are emitted
in a single pulse?

Answer 1

The total energy of pulse

$E=P\tau=N\frac{hc}{\lambda}$

Hence

$N=\frac{P\tau\lambda}{hc}\\ =\frac{12\times 16\times 10^{-3}\times 400\times 10^{-9} }{6.63\times 10^{-34}\times 3\times 10^8}=3.86\times 10^{17}$