Green light can have a wavelength of 543 nm. the energy of a photon of this light is

The energy per photon can be calculated through this formula:

E = hc/λ

where

h 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 543x10⁻⁹ m.

Substituting the values,

E = (6.626x10⁻³⁴ J·s) (3x10⁸ m/s) / (543x10⁻⁹ m)

E = 3.66x10⁻¹⁹ J/photon