Q 12-11-042JEE MainJEE Main 2026 (28 Jan, Shift 2)Easy
Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is ______.
(Given : $h = 6.63\times10^{-34}$ J.s and $c = 3\times10^8$ m/s)
Answer: (D) $2\times10^{16}$
Energy of one photon:
$$E = \frac{hc}{\lambda} = \frac{6.63\times10^{-34}\times3\times10^8}{663\times10^{-9}} = 3\times10^{-19}\ \text{J}$$
$$n = \frac PE = \frac{6\times10^{-3}}{3\times10^{-19}} = 2\times10^{16}\ \text{per second}$$
Solution by Sreeraj P, M.Sc Physics