How do you calculate the number of photons?

1 Answer
Ernest Z.
Aug 22, 2014

You calculate the energy of a photon, and then you use the total energy to calculate the number of photons.

To calculate the energy of a photon, see How do you calculate the energy of a photon of electromagnetic radiation?.

Then

Number of photons = Total energyEnergy of one photon

Few instructors will make the question so simple. Instead, they might disguise it as follows.

EXAMPLE

A common laser pointer produces 1.0 mW at a wavelength of 670 nm. Calculate the number of photons produced per millisecond.

Solution

Step 1. Calculate the energy of a photon.

E=hcλ=6.626×1034J⋅s×2.998×108m⋅s−1670×10−9m=2.965×10−19J

( 3 significant figures + 1 guard digit)

Step 2. Calculate the total energy per second.

Total energy = 1.0×10−3W×1J⋅s−11W=1.0×10−3J⋅s−1

Step 3. Calculate the number of photons per millisecond.

Number of photons =

Total energyEnergy of one photon=1.0×10−3J⋅s−1×1photon2.965×10−19J×

1s1000ms=3.4×1012photons/ms

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