Photochemistry, the branch of chemistry which studies the effect of light on chemical reactions. The light may be visible light, infrared radiation, or ultraviolet radiation. The chemical reaction takes place when the molecules of a substance change photochemically due to the light it absorbs.

Light is absorbed in photons, which are tiny packets of radiant energy. This energy is dependent on the wavelength of the light. In most cases, a molecule, after an increase in its energy due to absorbing photons, remains in an excited state for a millionth of a second or less. However, sometimes the molecule returns to its normal state by releasing the energy in the form of light or by losing the energy as a result of collision with other molecules. Sometimes, the molecule gains enough energy to undergo unusual chemical reactions while in the excited state. These reactions occur when the molecule absorbs ultraviolet light, which has a short wavelength.

Many photochemical reactions take place in nature. The most widely studied photochemical reaction is photosynthesis, the production of carbohydrates by plants and certain other organisms in the presence of light. The word photosynthesis means putting together with light. In photosynthesis, green plants make food using sunlight to combine carbon dioxide and water. Therefore, plants convert light energy into the chemical energy of food. Plants can also be converted into coal or petroleum through geological processes. The light energy stored in plants from millions of years is released when these fuels are burned.

Photochemical changes are also part of industrial processes, such as the formation of images in photography. When a picture is taken, some of the silver salts on the photographic film absorb light and chemically change into metallic silver, which produces a dark image on the negative when the film is developed.

The techniques used in photochemistry include those of analytical chemistry and of spectroscopy (the production and examination of spectra). Some of the techniques involve the use of lasers to initiate, change, or monitor chemical reactions.

The ongoing research in photochemistry is focused on the development of technological uses of solar energy. Photochemists are trying to find ways to imitate the process of photosynthesis with artificially created molecules. They hope to convert sunlight into electricity in a more efficient way than the methods currently used. Many photochemists are studying ways of using sunlight to produce such fuels as hydrogen gas and methanol. Some of these methods use solar photons to split molecules of water.