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

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