The number of photons emitted by a 5mW laser source operating at 632.8 nm is
The energy of each photon is
=E hcλ=(6.63×10−34J s)×(3×108ms−1)(632.8×10−9m)=3.14×10−19J.The energy of the laser emitted per second is 5×10−3J.Thus the number of photons emitted per second=5×10−3J)3.14×10−19J=1.6×1016.