Question

A certain Ruby laser emits 100J pulses of light whose wavelength is 6940A.What is the minimum no. of Cr3+ ions in the laser?

Answer 1

$$n=\frac{E}{E_\gamma}\\ \\E=\frac{hc}{\lambda}=\frac{1.986\cdot10^{-25}}{(694\cdot10^{-9})}=2.86\cdot10^{-19}J\\n=\frac{100}{2.86\cdot10^{-19}}=3.5\cdot10^{20}$$