There are several different methods for bringing about a chemical reaction: these include introducing heat, applying pressure, or simply adding one chemical to another. Some chemical reactions only occur when the reactants are exposed to light; one example of such a reaction is photography, which relies upon light to create a chemical reaction in the film. Reactions in which light is used as a catalyst are called photochemical reactions, and the study of such reactions is referred to as the science of photochemistry.

Photochemical reactions occur in two stages: primary and secondary. In a primary reaction, light is absorbed by a molecule; this will either cause the molecule to be fragmented, or will excite one of its electrons from a lower orbit to a higher one. The secondary stage determines what happens next. If the molecule was fragmented, the fragments will either recombine (producing the original molecule) or...