The power of the laser is
[tex]P=16 mW=0.016 W[/tex]
by using the relationship between power, energy and time, we can find the energy delivered by the laser in 1 second:
[tex]E=Pt=(0.016 W)(1 s)=0.016 J[/tex]
The frequency of the photons of this light is given by
[tex]f= \frac{c}{\lambda}= \frac{3\cdot 10^8 m/s}{632.8 \cdot 10^{-9} m} =4.74 \cdot 10^{14} Hz [/tex]
and the energy of a single photon is
[tex]E_1=hf=(6.6\cdot 10^{-34} Js)(4.74 \cdot 10^{14} Hz)=3.13 \cdot 10^{-19} J[/tex]
In order to find the number of photons emitted per second, we must divide the total energy emitted by the laser by the energy of a single photon, and we get:
[tex]N= \frac{E}{E_1}= \frac{0.016 J}{3.13 \cdot 10^{-19}J} =5.11 \cdot 10^{16} photons [/tex]
