Photochemistry is a branch of physical chemistry dealing with chemical reactions that produce light (see chemiluminescence and combustion), or that are initiated by light (visible or ultraviolet). Important examples include photosynthesis, photography, and bleaching by sunlight.
One photon of light, of a suitable wavelength, may be absorbed by a molecule, raising it to an electronically excited state. Re-emission may occur by fluorescence or phosphorescence, the energy may be transferred to another molecule, or a reaction may occur, commonly dissociation to form free radicals. The quantum yield, or efficiency, of the reaction is the number of molecules of reactant used (or product formed) per photon absorbed; this may be very large for chain reactions.
See also photochemical.