Monochromatic light of wavelength 632.8 nm is produced by a helium-neon laser. The power emitted is 9.42mW.
How many photonsper second, on the average, arrive at a target irradiated by beam? (Asume the entire beam to be incident on a small area of the target)

Explanation

Emission power, `P = 9.42 mW = 9.42 xx 10^(-3)W`
`:.` Number of photons emitted per second,
`n = (P)/(E) = (9.42 xx 10^(-3))/(3.14 xx 10^(-19)) = 3 xx 10^(16)`