In this example problem, we calculate the number of photons in a laser pulse. Our problem reads: calculate the number of photons in a 1.75 millijoules er pulse which has a wavelength of 2.1 micrometers. Our first step will be to calculate the energy of one photon from the laser so that we can use this as a conversion factor in dimensional analysis to convert from millijoules into photons. To calculate the energy of one photon, we use the equation E = HC/λ, where H is Planck's constant, C is the speed of light, and λ is our wavelength in meters. Our wavelength is given in micrometers, so we can convert 2.1 micrometers into meters and substitute our numbers into the equation. We have Planck's constant (6.6 * 10^-34 joule-seconds) times the speed of light (3 * 10^8 meters per second) divided by the wavelength (2.10 * 10^-6 meters). Completing this calculation, we should get that the energy of one photon is 9.466 * 10^-20 joules. Now, we'll use this as a conversion factor in our dimensional analysis to calculate the number of photons. We start our dimensional analysis with 1.75 millijoules and we can convert this into joules. Then, we can convert from joules to photons using the value we calculated in step one (one photon = 9.466 * 10^-20 joules). Completing this calculation, we should get 1.85 * 10^16 photons.