How many photons are produced in a laser pulse of 0.258 j at 683 nm?

We can calculate the energy of a single photon through Plank-Einstein relation. We have



where is the Plank's constant and is the frequency. Also, recall that to solve for the frequency, we have



where is the speed of light and is the wavelength of the  laser, in meters. So for this problem we can compute for with



Going back to the Planck-Einstein relation, we have



Hence, we have .

Given that the laser emits an energy of 0.258 J, then there are

.

7.5 ⋅ 10^18 photons