A laser pulse with wavelength 555 nm contains 4.40 mj of energy. how many photons are in the laser pulse?

Wavelength is the ratio of speed of light and frequency:

wavelength = speed of light / frequency

where frequency is: (c is speed of light)

freq = c / wavelength = 3E8/550E-9 = 5.45E14 Hz

We know that photons have energy E = hf = 6.626E-34 Js * 5.45E14 Hz = 3.61 E-19 J

Therefore if it has 4.40 mJ of energy, the pulse has:

4.40E-3/3.61E-19 = 1.22E16 photons