Question

A $1.0mW$ helium-neon laser emits a visible laser beam with a wavelength of $633nm$. How many photons are emitted per second?

Short answer

No of photons emitted per second are

$N=3.34\times {10}^{15}$

Step-by-step solution

Given information

We have given that,

$$\begin{gathered} P=1mW={10}^{-3}W \\ \lambda =633nm=633\times {10}^{-9}m \end{gathered}$$

We have to find the no, of photons emitted per second.

Simplification

Since $P=0.0010W$, the laser light emits $E_{light}=0.0010J$of light energy per second.

This energy consists of $N$photons. The energy of each photon is

$E=hf=\frac{hc}{\lambda }=3.14\times {10}^{-19}J$

As $E_{light}=N{E}_{photon}$,the no. of photons is

$N=\frac{E_{light}}{E_{photon}}=\frac{0.0010J}{3.13\times {10}^{-19}J}=3.2\times {10}^{15}$

So, the photons are emitted at the rate of$3.2\times {10}^{15}{s}^{-1}$.