Question

A monochromatic source of light operating at 200 W emits $4\times {10}^{20}$ photons per second. The wavelength of this light will be __ nm.

Answer 1

$\text{The energy of each photon is}=\frac{200J{s}^{-1}}{4\times {10}^{20}{s}^{-1}}=5\times {10}^{-19}J.Wavelength=\lambda =\frac{hc}{E}=\frac{(6.63\times {10}^{-34}Js)\times (3\times {10}^{8}m{s}^{-1})}{(5\times {10}^{-19}J)}=4.0\times {10}^{-7}m=400nm.$