A  $2 \mathrm{mW}$  laser operates at a wavelength of $500 \mathrm{nm}$. The number of photons that will be emitted per second is

[Given Planck's constant  $h=6.6\times {10}^{-34} \mathrm{Js}$, speed of light  $c=3.0\times {10}^{8 }{\mathrm{ms}}^{-1}$]

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

Let number of photons per second be $n$.

Power of laser = P = 2mW = $2\times {10}^{-3} \mathrm{W}$

Wavelength = $\lambda =500 \mathrm{nm}=500\times {10}^{-9} \mathrm{m}$

$$\begin{gathered} \text{∴ Total power }n\left(\frac{hc}{\lambda }\right)=P \\ \Rightarrow n=\frac{{\displaystyle P\lambda }}{{\displaystyle hc}}=\frac{{\displaystyle \left(2\times {10}^{-3}\right)\left(500\times {10}^{-9}\right)}}{{\displaystyle \left(6.6\times {10}^{-34}\right)\left(3\times {10}^8\right)}}\approx 5\times {10}^{15}\text{ photons/sec } \end{gathered}$$