How many photons are emitted per second by a 5 mW laser source operating at 632.8 nm?

$\begin{array}{l}E=\frac{hc}{\lambda }=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{632.8\times {10}^{-9}}\mathrm{J}\\ \text{ Number of photons per second, }n=\frac{P}{E}\\ \Rightarrow n=\frac{5\times {10}^{-3}}{3.14\times {10}^{-19}}=1.6\times {10}^{16}\end{array}$