Monochromatic light of frequency 6.0 x ${10}^{14}$ Hz is produced by a laser. The power emitted is 2 x ${10}^{-3}$ W. The number of photons emitted. on the average, by the source per second is

Photons are the packets of energy. power emitted,

$\begin{array}{l}\mathrm{P}=2\times {10}^{-3}\mathrm{W}\\ \mathrm{Energy}\mathrm{of}\mathrm{photon},\mathrm{E}=\mathrm{hv}=6.6\times {10}^{-34}\times 6\times {10}^{14}\mathrm{J}\\ \mathrm{h}\mathrm{being}\mathrm{Plank}\text{'}\mathrm{s}\mathrm{constant}.\\ \mathrm{Number}\mathrm{of}\mathrm{photons}\mathrm{emitted}\mathrm{per}\mathrm{second}\\ \mathrm{n}=\frac{\mathrm{P}}{\mathrm{E}}=\frac{2\times {10}^{-3}}{6.6\times {10}^{-34}\times 6\times {10}^{14}}=5\times {10}^{15}\end{array}$