how many photons are emitted per second by a 5mw laser source operating at 632.8nm?

Dear Student

Energy, E of the photon is given by -

E = $\frac{hc}{\lambda }$
So, E = $\frac{6.63 x {10}^{-34} Js x 3 x {10}^8 m/s}{632.8 x {10}^{-9} m} = 3.14 x {10}^{-19} J$

Now, the energy emitted per second by the laser source = 5 mW or 5 x 10^-3 J

Thus, the total number of photons emitted per second = $\frac{5 x {10}^{-3} J}{3.14 x {10}^{-19} J} = 1.6 x {10}^{16}$ 

Regards