The branch of chemistry concerned with the effect of absorption of radiant energy (light) in inducing or modifying chemical changes. Photosynthesis is the most important example of a photochemical reaction; others are the photosensitization of solids, applied in photography, photocells, photovoltaic cells, and the formation of visual pigments; photochemical decomposition(photolysis), photo-induced polymerization, oxidation, and ionization; fluorescence and phosphorescence; and the reaction of chlorine with organic compounds. Free-radical chain mechanisms are usually involved.See Free Radical.