[UPSC IFoS 2021] Polarisation and Modern Optics (Q341, 8M)

Consider that a continuous laser beam which is assumed to be perfectly monochromatic with a wavelength of 6500\text{ \AA} is chopped into 2\text{ ns} pulses using a shutter. Calculate the number of photons in each pulse, if the power of each pulse is 5\text{ mW}.