To determine the number of photons that is being emitted in a certain light, we need to know the total power of the system and the energy of one photon. Usually, in problems like this, the power of the system is given. We just need to calculate for the energy of a single photon which is equal to hc/wavelength where h is the Planck's constant and c is the speed of light. We divide the total power with the energy of one photon then we obtain the number of photons per second. For instance, we are given the following:
P = 100 W
wavelength = 400nm
Energy of one photon = 6.626x10^-34 x 3x10^8 / 400x10^-9 = 4.969x10^-19 J
Np = 100 / 4.969x10^-19 = 2.012x10^20 per second
