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 required for one 16-ms pulse is power times duration:

$$E=Pt.$$

The energy of a flux of N photons with wavelength λ each:

$$E=\frac{Nhc}{\lambda}.$$

These energies are equal assuming zero energy losses. Express and calculate N:

$$N=\frac{Pt\lambda}{hc}=3.9\cdot10^{17}.$$