Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is $

Question

Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is $\_\_\_\_$ . (Given : $h=6.63 	imes 10^{-34} \mathrm{~J} . \mathrm{s}$ and $\mathrm{c}=3 	imes 10^8 \mathrm{~m} / \mathrm{s}$ )

Professor PC Thomas Classes · Answer

The energy of a single photon is given by $E = \frac{hc}{\lambda} = \frac{6.63 	imes 10^{-34} 	imes 3 	imes 10^8}{663 	imes 10^{-9}} = \frac{19.89 	imes 10^{-26}}{6.63 	imes 10^{-7}} = 3 	imes 10^{-19}$ J. The power of the laser is $P = 6$ mW $= 6 	imes 10^{-3}$ J/s. The number of photons emitted per second is $n = \frac{P}{E} = \frac{6 	imes 10^{-3}}{3 	imes 10^{-19}} = 2 	imes 10^{16}$.