Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light or infrared radiation. Photochemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules' absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules.hotosynthesis: plants use solar energy to convert carbon dioxide and water into glucose and oxygen. Human formation of vitamin D by exposure to sunlight. ... Vision is initiated by a photochemical reaction of rhodopsin.