Question

A laser pulse with wavelength 520 nm contains 4.85 mJ of energy. How many photons are in the laser pulse?

Answer 1

wavelength = 520 nm

frequency v = C / lambda

                     = 3 x 10^8 / 520 x 10^-9

                      = 5.77 x 10^14 Hz

energy = 4.85 mJ = 4.85 x 10^-3 J

E = n h v

4.85 x 10^-3 = n x 6.625 x 10^-34 x 5.77 x 10^14

n = 1.27 x 10^16

number of photons = 1.27 x 10^16