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

Given, `lambda = 632.8 nm = 632.8 xx10^(-9)m, p = 9.42 xx10^(-3)W`
`h=6.63xx10^(-34)J-s, c=3xx10^(8)m//s`
No. of photons per second, `N = ("Power (P)")/("Energy (E )")=(9.42xx10^(_3))/(3.14xx10^(-19)) therefore N = 3xx10^(16)` photons/s