Question

Monochromatic light of wavelength 632.8 nm is produced by a helium-neon laser. The power emitted is 9.42 mW.
Find the energy and momentum of each photon in the light beam,

Answer 1

$\lambda =632.8nm=632\times {10}^{-9}m$
power, $P=9.42mW=9.42\times {10}^{-3}W$
Energy of each photon in the light beam,
$E=\frac{hc}{\lambda }=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{632.8\times {10}^{-19}}=3.14\times {10}^{-19}$
$=\frac{3.14\times {10}^{-19}}{1.6\times {10}^{-19}}=1.963eV$
The momentum of each photon in the light beam,
$p=\frac{h}{\lambda }=\frac{6.6\times {10}^{-34}}{632.8\times {10}^{-19}}=1.046\times {10}^{-27}kgm{s}^{-1}$