A monochromatic light of frequency $3 \times {10}^{14} \mathrm{Hz}$ is produced by a LASER, emits the power of $3 \times {10}^{-3} \mathrm{W}.$ Find how many number of photons are emitted per second.

Energy of a photon, $\mathrm{E} = \mathrm{h}\nu$

$$\begin{gathered} \mathrm{E} =\left(6.63 \times {10}^{-34} \mathrm{J} \mathrm{s}\right) \left(3 \times {10}^{14} \mathrm{Hz}\right) \\ = 19.89 \times {10}^{-20} \mathrm{J} \end{gathered}$$

Power of the source, $\mathrm{P} = 3 \times {10}^{-3} \mathrm{W}$

Number of photons emitted per second,

$\mathrm{N} =\frac{\mathrm{P}}{\mathrm{E}}=\frac{3 \times {10}^{-3} \mathrm{W}}{19.89 \times {10}^{-20} \mathrm{J}}= 1.5 \times {10}^{16}$