Hadronic scattering experiments produced extensive and rich data revealing resonances and regularities of cross sections. While the quark model provided a firm basis for classifying the particles and resonances, the scattering cross sections were less easily interpreted. The early studies of strong interactions indicated that the couplings of the particles were large. This precluded the straightforward use of perturbation theory. While alternative approaches have yielded some important results, it is still true that even processes as basic as elastic proton–proton scattering are beyond our ability to explain in detail. In contradistinction, scattering of electrons by protons and neutrons is open to direct interpretation.

For the scattering of an electron by a proton it is a good approximation to assume that the interaction is due to the exchange of a single virtual photon. The small corrections to this approximation may be calculated if necessary. Each coupling of the photon gives a factor of e in the scattering amplitude, so a virtual photon's two couplings typically provides a factor α = e2/4π ≈ 1/137. It is this small number that makes the approximation a good one.

The scattering of relativistic electrons (E > > me) by a known charge distribution can be calculated using the standard methods of quantum mechanics.