Q 12-11-111JEE MainJEE Main 2022 (26 Jul, Shift 1)Medium
A parallel beam of light of wavelength $900\ \text{nm}$ and intensity $100\ \text{W m}^{-2}$ is incident on a surface perpendicular to the beam. The number of photons crossing $1\ \text{cm}^2$ area perpendicular to the beam in one second is
Answer: (B) $4.5\times10^{16}$
Energy per photon: $E = \dfrac{hc}{\lambda} = \dfrac{6.63\times10^{-34}\times3\times10^8}{900\times10^{-9}} = 2.21\times10^{-19}\ \text{J}$.
Power through $1\ \text{cm}^2$: $100\times10^{-4} = 10^{-2}\ \text{W}$.
$$N = \frac{10^{-2}}{2.21\times10^{-19}}\approx4.5\times10^{16}\ \text{s}^{-1}$$
Solution by Sreeraj P, M.Sc Physics