Photochemistry is the branch of chemistry which deals with the effect of light on chemical systems. A photochemical reaction is caused by the absorption of electromagnetic radiation, especially ultraviolet (UV)—visible radiations. Photochemical reactions proceed differently from thermal reactions; hence they have a lot of importance in organic and inorganic chemistry. The photochemical path offers advantages over thermal methods of forming thermodynamically disfavoured products by overcoming large activation barriers and the reaction will be completed in a short period of time. Hence, many thermal reactions have photochemical counterparts.
Some examples are photosynthesis, formation of Vitamin D with sunlight ...