Photon Substructure

Technically speaking, photons are fundamental...a photon is a photon is a photon. However, at times they can behave as though they have substructure. If a photon splits into a virtual quark-anti-quark pair just as it's taking part in a hard scattering process, it looks as if photon has substructure; also, the photon can mix with other particles that have the same quantum numbers, a rho meson for example, and if that other particle has hadronic substructure, so does the photon. The latter process is known as vector meson dominance