Energy (E) of a photon = hν

Energy (En) of ‘n’ photons = nhν

Where, λ = wavelength of light = 4000 pm = 4000 ×10^{–12} m c = velocity of light in vacuum = 3 × 10^{8} m/s h = Planck’s constant = 6.626 × 10^{–34} Js

Substituting the values in the given expression of n:

Hence, the number of photons with a wavelength of 4000 pm and energy of 1 J are

2.012 × 10^{16}.