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

Electrical energy of the laser:

$$E=Pt.$$

Energy o the emitted photons:

$$E=n\frac{hc}{\lambda}.$$

Hence:

$$Pt=n\frac{hc}{\lambda},\\\space\\ n=\frac{Pt\lambda}{hc}=3.866\cdot10^{17}.$$