Question

A laser pulse with wavelength 545 nm contains 4.85 mJ of energy. How many photons are in the laser pulse?

Answer 1

Given:

Wavelength (λ) of the laser pulse = 545 nm = 5.45 * 10⁻⁹ m

Total energy of pulse = 4.85 mJ

To determine:

The number of photons in the laser of a given energy

Explanation:

Energy per photon (E) = hc/λ

where h = planck's constant = 6.626 *10⁻³⁴ Js

C = speed of light = 3*10⁸ m/s

λ = wavelength

E = 6.626 *10⁻³⁴ Js* 3*10⁸ms-1 /5.45 * 10⁻⁹ m = 3.65 * 10⁻¹⁹ J

Now,

# photons = total energy/Energy per photon

                 =  4.85 * 10⁻³ J* 1 photon / 3.65 * 10⁻¹⁹ J = 1.32 * 10¹⁶ photons

Ans: the laser pulse contains 1.32 * 10¹⁶ photons