Q 12-11-160JEE MainJEE Main 2019 (10 Apr, Shift 2)Easy
A $2\ \text{mW}$ laser operates at a wavelength of $500\ \text{nm}$. The number of photons that will be emitted per second is: [Given Planck's constant $h = 6.6\times10^{-34}\ \text{J s}$, speed of light $c = 3.0\times10^8\ \text{m/s}$]
Answer: (B) $5\times10^{15}$
Photon energy $E = \dfrac{hc}{\lambda} = \dfrac{6.6\times10^{-34}\times3\times10^8}{500\times10^{-9}} = 3.96\times10^{-19}\ \text{J}$.
$$N = \frac{2\times10^{-3}}{3.96\times10^{-19}} \approx 5\times10^{15}\ \text{per second}$$
Solution by Sreeraj P, M.Sc Physics