Electron-proton collision. Artwork illustrating a collision event between an electron and a proton within the nucleus of a deuterium atom. The nucleus is depicted as collections of down (blue) and up (red) quarks: the proton has two up and one down whilst the neutrons each have two down and one up quark. The colours around the quarks represent aspects of the strong nuclear force binding the quarks together. The electron (orange) approaches the nucleus - its track shown in yellow. As it nears the proton (below centre), it interacts with the proton's down quark (blue) by the exchange of a virtual photon (yellow wavy line).