Monochromatic light of wave length $632.8\;nm$ is produced by a helium - neon laser. The power emitted is $9.42\;mw$. How many photons per energy, on the average, arrive at a target irradiated by this beam ? (Assume the beam to have uniform cross - section which is less than the target area )

Question

$\begin{array}{1 1} 3 \times 10^{6}\; photons /sec \\ 8 \times 10^{6}\; photons /sec \\ 3 \times 10^{-6}\; photons /sec \\ 5 \times 10^{6}\; photons /sec\end{array} $