Monochromatic light of wave length $632.8\;nm$ is produced by a helium - neon laser. The power emitted is $9.42\;mw$. How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon .

Question

$\begin{array}{1 1} 6.3 ms^{-1} \\ 6.3 \;kms^{-1} \\ 0.63 ms^{-1} \\ 0.63kms^{-1} \end{array}$