How many photons are emitted per second by a $5 \mathrm{mW}$ laser source operating at $663 \mathrm{nm}$?

The energy of each photon is

$E=\frac{hc}{\lambda }$$=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{663\times {10}^{-9}}$$=3\times {10}^{-19} \mathrm{J}$

The energy of the laser emitted per second is $5\times {10}^{-3} \mathrm{J}.$ Thus, the number of photons emitted per second is

$n=\frac{P}{E}$$=\frac{5\times {10}^{-3}}{3\times {10}^{-19}}=1.67\times {10}^{16} \mathrm{photons}/\sec$