Photochemistry is the study of chemical changes involving light. It studies those processes in which light causes chemical changes. Also it studies the reverse processes in which chemicals react to emit light. Photo is from Greek meaning "light." Photochemical processes of the first type are illustrated by what happens to a film when a picture is being taken, and by the whitening of dark-colored clothing or hair when exposed to sunlight. Examples of the second type are illustrated by the flashing of a firefly and the burning of candles or oil and gas lamps.
In nature, an important, photochemical change occurs in green plants. CHLOROPHYLL in the cells of green plants uses light to combine ordinary water and carbon dioxide chemically to make sugar. This photochemical process is called PHOTOSYNTHESIS.
Photochemistry is basic to making and developing of photographic FILM. A clear plastic film is coated with gelatine containing tiny grains of silver bromide. When the film is placed in a camera and light is focused on it through the shutter and lens, the silver bromide undergoes chemical changes in those spots where light strikes it. The grains in these spots are said to be sensitized. Chemical developers reduce the sensitized grains of silver bromide to metallic silver. Chemicals called fixers are next used to remove the silver bromide which was not sensitized.