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),

Explanation

Here, `lamda=632.8nm=632.8xx10^(-9)m`,
`P=9.42mW=9.42xx10^(-3)W`
Number (n) of photons falling per second on the target is
`n=(P)/(E)=(9.42xx10^(-3))/(3.14xx10^(-19))=3xx10^(16)` photons/sec