# How do you calculate the number of photons?

##### 1 Answer

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 =

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.

( 3 significant figures + 1 guard digit)

**Step 2.** Calculate the total energy per second.

Total energy =

**Step 3.** Calculate the number of photons per millisecond.

Number of photons =