Monochromatic light of frequency $6.0 \times 10^{14} \;Hz$ is produced by a laser. The power emitted is $2.0 \times 10^{-3} \;W$.
$(a)$ What is the energy of a photon in the light beam?
$(b)$ How many photons per second,on an average,are emitted by the source?

(N/A) The energy of each photon is given by $E = h\nu$,where $h = 6.63 \times 10^{-34} \;J \cdot s$ and $\nu = 6.0 \times 10^{14} \;Hz$.
$E = (6.63 \times 10^{-34} \;J \cdot s)(6.0 \times 10^{14} \;Hz) = 3.978 \times 10^{-19} \;J \approx 3.98 \times 10^{-19} \;J$.
$(b)$ If $N$ is the number of photons emitted per second,the total power $P$ is given by $P = N \cdot E$.
Therefore,$N = \frac{P}{E} = \frac{2.0 \times 10^{-3} \;W}{3.978 \times 10^{-19} \;J} \approx 5.03 \times 10^{15} \;\text{photons/s}$.