Photochemistry is a branch of chemistry that deals with the chemical reactions that result when the molecules of a substance absorb light. A molecule changes photochemically only if it absorbs light, not if light passes through it or is reflected.
Light is absorbed in tiny packets of radiant energy called photons. The energy of a photon depends on the wavelength of the light. After absorbing a photon, a molecule increases in energy and is in an excited state. In most cases, the molecule remains in that state only a millionth of a second or less. Sometimes the molecule returns directly to its normal state by releasing the energy as light or by losing the energy in collisions with other molecules. At other times, the molecule has gained enough energy to undergo unusual chemical reactions while in the excited state. These reactions occur when the molecule absorbs visible or ultraviolet light, which has a short wavelength.
Photochemical reactions are part of many natural processes. In photosynthesis, for example, green plants absorb sunlight. The plants use this light energy to make food out of carbon dioxide from the air and water from the soil (see Photosynthesis ). Plants thus convert light energy into the chemical energy of food. Through geological processes, plants may be converted into coal or petroleum. As these fuels are burned, the light energy stored in the plants millions of years before is released.
Many industrial processes also involve photochemical changes. In photography, for example, some of the silver salts in photographic film absorb light when a picture is taken. The absorbed light chemically changes these salts into metallic silver. When the film is developed, the metallic silver produces a dark image on the negative.
Much research in photochemistry today involves the development of technological uses of solar energy. Some photochemists are seeking ways to imitate the process of photosynthesis with artificially created molecules. These chemists hope to convert sunlight into electricity in a more efficient way than is now possible. Other photochemists are studying methods for using sunlight to produce such fuels as hydrogen gas and methanol. Some of these methods involve splitting molecules of water with solar photons.
