Monochromatic light of wavelenght `667nm` is produced by a helium neon laser. The power emitted is `9mW`. The number of photons arriving per seconid on the average at a target irradiated by this beam

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