Photons Questions

Monochromatic light of frequency $6.0\times 10^{14}\text{Hz}$ is produced by a laser. The power emitted is $2\times 10^{-3}\text{W}$. The number of photons emitted, on the average, by the source per second is:

Monochromatic light of wavelength $667\text{nm}$ is produced by a helium neon laser. The power emitted is $9\text{mW}$. The number of photons arriving per sec. on the average at a target irradiated by this beam is:

A $200\text{W}$ sodium street lamp emits yellow light of wavelength $0.6\mu \text{m}$. Assuming it to be $25\%$ efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is: