Question

Q2. 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 energy of each photon with wavelength $\lambda$:

$$E_1=h\nu=h\frac{c}{\lambda}.$$

The electrical energy required to provide 12 W during 16 ms:

$$E=Pt.$$

This energy will be enough for N photons:

$$Pt=N\cdot h\frac{c}{\lambda},\\ \space\\ N=\frac{Pt\lambda}{hc}=\frac{12(16\cdot10^{-3})(400\cdot10^{-9})}{(6.626\cdot10^{-34})(3\cdot10^8)}=3.87\cdot10^{17}.$$