Question

A laser pulse with wavelength 540 nm contains 4.85 mj of energy. how many photons are in the laser pulse?

Answer 1

The energy per photon can be calculated through this formula:

E = hc/λ
where
ħ is Planck's constant = 6.626x10⁻³⁴ J·s 
c is the speed of light = 3x10⁸ m/s 
λ is the wavelength

The wavelength is given to be 540 nm or also equivalent to 540x10⁻⁹ m. Substituting the values,
E = (6.626x10⁻³⁴ J·s )(3x10⁸ m/s )/(540x10⁻⁹ m)
E = 1.681x10⁻¹⁹ J/photon

The total energy is given to be 4.85 mj or also equal to 4.85x10⁻³ J. Thus,

Number of Photons = 4.85x10⁻³ J/1.681x10⁻¹⁹ J/photon
Number of Photons = 1.32x10¹⁶