There are two models for radiant energy, the particle model and the wave model. In the particle model, radiant energy is comprised of discrete packets called photons. A photon's electromagnetic energy is defined as:

E_photon = hf, where h is Planck's constant and f is the frequency of the electromagnetic energy

The energy amplitude measured at a specific wavelength of frequency f is determined by the number of photons at that wavelength, i.e., amplitude for the wave model is equivalent to numbers of photons for the particle model.