Question

A laser pulse with wavelength 520 nm contains 4.40 mj of energy. how many photons are in the laser pulse?

Answer 1

Given:
λ = 520 nm = 520 x 10⁻⁹ m, the wavelength
E = 4.4 mJ = 4.4 x 10⁻³ J, energy per packet or pulse

The Planck-Einstein equation is
E = (hc)/λ
where
c = 3 x 10⁸ m/s, the velocity of light

The energy per photon is
e = fh
where 
f = c/λ = 3 x 10⁸/520 x 10⁻⁹ = 5.7692 x 10¹⁴ 1/s

The number of photons is
E/e = c/(fλ)
      = (3 x 10⁸)/[(5.7692 x 10¹⁴)*(520 x 10⁻⁹)]
      = 1

Answer:  1