Question

Light of wavelength$5000A^0$ falls on a sensitive surface. If the surface has received ${10}^{-7}$ J energy, how many photons have fallen on the surface?

Answer 1

Wave length of a photon $\lambda =5000\mathring{A}=5000\times {10}^{-10}m$

Rate at which photons emitted$={10}^{-7}J/s$

$n=\frac{E}{hc/\lambda }$

$=\frac{{10}^{-7}}{6.6\times {10}^{-34}\times 3\times {10}^8/5000\times {10}^{-10}}=252\times {10}^9$ photons.