Question

The number of photons emitted by a 5mW laser source operating at 632.8 nm is __$\times {10}^{16}$

Answer 1

The energy of each photon is

$=E\frac{hc}{\lambda }=\frac{(6.63\times {10}^{-34}Js)\times (3\times {10}^{8}m{s}^{-1})}{(632.8\times {10}^{-9m})}=3.14\times {10}^{-19}J.\text{The energy of the laser emitted per second is}5\times {10}^{-3}J.\text{Thus the number of photons emitted per second}=\frac{5\times {10}^{-3}J)}{3.14\times {10}^{-19}J}=1.6\times {10}^{16}.$