# How many photons are produced in a laser pulse of 0.169 J at 439 nm?

## Expert Answers A laser pulse of a given energy E will contain n photons, each with the energy hf, where h is the Planck constant and is the frequency of the photons in the light pulse. The value of the Planck constant is

`h=6.626*10^(-34)` J.

The frequency of a photon is related to its wavelength, `lambda`  ,by

`lambda = c/f` , where c is the speed of light, `3*10^8 m/s`

So the frequency of the photons in the given light pulse is

`f=c/lambda = (3*10^8)/(439*10^(-9) )= 6.83*10^14 1/s`

Then, the energy of each such photon is

`hf = 6.626*10^(-34) * 6.83*10^14 =4.53*10^(-19)` J.

Then, the number of the photons in the light pulse is

`n=E/(hf) = 0.169/(4.53*10^(-19)) = 3.73*10^17`

or 373,000,000,000,000,000.

The number of photons in the light pulse is 373,000,000,000,000,000.

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