Monochromatic light of wavelength 632.8 nm is produced by a helium-neon laser. The power emitted is 9.42 mW. How many photons per second, 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

$\begin {array} {1 1} (a)\;0.003 \times 10^{19}photon/s & \quad (b)\;30 \times 10^{16}photon/s \\ (c)\;3 \times 10^{15}photon/s & \quad (d)\;0.33 \times 10^{16}photon/s \end {array}$