Question

How many photons are emitted per second by a He−Ne laser that emits 1.5 mW of power at a wavelength λ=632.8nm. What is the frequency of the electromagnetic waves emitted by a He−Ne laser?

Answer 1

The frequency of the emitted EM wave by He-Ne laser is
`4.74* 10^(14) Hz`

Given:

Power emitted by He-Ne laser, P = 1.5 mW =
`1.5* 10^(- 3)`

Wavelength,
`\lambda = 632.8 nm = 632.8* 10^(- 9) m`

Solution:

Now, to calculate the frequency,
`\vartheta` of the EM wave emitted:

Energy associated with 1 photon = Power of one photon per sec =
`(hc)/(\lambda)`

Therefore, power associated with 'N' no. of photons, P =
`N(hc)/(\lambda)`

where

`h = 6.626* 10^(-34) m^(2)kg/s` = Planck's constant

Now,

`1.5* 10^(- 3) = N(6.626* 10^(-34)* 3* 10^(8)))/(632.8* 10^(- 9))`

N =
`4.77* 10^(15)`

Also, we know that:

`c = \vartheta \lambda`

Thus

`\vartheta = (c)/(\lambda) = (3* 10^(8))/(632.8* 10^(- 9))`

`\vartheta = 4.74* 10^(14) Hz`