Question

A helium-neon laser emits light of wavelength 632.8 nm and has a power output of 16 mw. how many photons are emitted per second by this laser?

Answer 1

The power of the laser is
$P=16 mW=0.016 W$
by using the relationship between power, energy and time, we can find the energy delivered by the laser in 1 second:
$E=Pt=(0.016 W)(1 s)=0.016 J$

The frequency of the photons of this light is given by
$f= \frac{c}{\lambda}= \frac{3\cdot 10^8 m/s}{632.8 \cdot 10^{-9} m} =4.74 \cdot 10^{14} Hz$
and the energy of a single photon is
$E_1=hf=(6.6\cdot 10^{-34} Js)(4.74 \cdot 10^{14} Hz)=3.13 \cdot 10^{-19} J$

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:
$N= \frac{E}{E_1}= \frac{0.016 J}{3.13 \cdot 10^{-19}J} =5.11 \cdot 10^{16} photons$