Question

A 25 watt bulb emits monochromatic yellow light of wavelength of $0.57\mu m$. Calculate the rate of emission of quanta per second.

Answer 1

The energy of one photon is

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

$=\frac{6.626\times {10}^{-34}Js\times 3\times {10}^{8}m/s}{0.57\times {10}^{-6}}$

$=3.487\times {10}^{-19}J$ .

25 watt corresponds to 25 J per second.

The number of photons emitted per second is $\frac{25}{3.487\times {10}^{-19}}=7.17\times {10}^{19}/s$.