Question

How many photons are emitted during 3.5 s of operation of a red laser pointer? The device outputs 2.99 Watts at a 620.85 nm wavelength.

Answer 1

Given:

The power of the laser is P = 2.99 W

The time duration is t = 3.5 s

The wavelength of the laser is

$\begin{gathered} \lambda\text{ = 620.85 nm} \\ =620.85\text{ }\times10^{-9}\text{ m} \end{gathered}$

Required: Number of photons emitted from the laser.

Explanation:

The energy of the laser is given by the formula

$E=\frac{nhc}{\lambda}$

Here, n is the number of photons.

h is the Planck's constant whose value is

$h\text{ = 6.6}\times10^{-34\text{ }}J\text{ s}$

c is the speed of light whose value is

$c=3\times10^8\text{ m/s}$

The energy can also be calculated as

$\begin{gathered} E=P\times t \\ =2.99\times3.5 \\ =10.465\text{ J} \end{gathered}$

On substituting the values, the number of photons can be calculated as

$\begin{gathered} n=\frac{E\lambda}{hc} \\ =\frac{10.465\times620.85\times10^{-9}}{6.6\times10^{-34}\times3\times10^8} \\ =3.28\times10^{19}\text{ } \end{gathered}$

Final Answer: 3.28e19 photons are emitted.