Now we have c = fλ. Rearranging, this gives f = c/λ.
Next from quantum theory, we are given that E = hf. This relates the energy of a photon, or particle of light, (E), and the frequency of the light (f), with the constant of proportionality ‘Planck’s Constant’ (h).
Combining this with the wave equation, we arrive at E = hc/λ.
Plugging in h = 6.6*10^-34Js^-1, c = 3.0*10^8ms^-1 and λ = 5.0m (all to two significant figures):
E = (6.6*10^-34)(3.0*10^8)/(5.0) = 4.0*10^26J per photon. I hope this helps you 🙂