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Photochemistry is the branch of science which deals with chemical reactions, reactions which occurred by the absorption of light waves which are called photons of energy. Only the radiations which fall under the region 800 nm to 200 nm can cause such reactions. Also the photons should contain sufficient amount of energy to raise the atoms from ground to excited state. In excited state atoms or groups of atoms undergoes chemical reaction more rapidly as compared to ground state. When a molecule absorbs a photon of light, its electronic structure changes, and it reacts differently with other molecules. The energy that is absorbed from light can result in photochemical changes in the absorbing molecule, or in an adjacent molecule (e.g., photosensitization). The energy can also be given off as heat, or as lower energy light, i.e., fluorescence or phosphorescence, in order to return the molecule to its ground state. Each type of molecule has a different preference for which of these different mechanisms it uses to get rid of absorbed photon energy, e.g., some prefer fluorescence over chemistry. Photochemical reactions can happen in a variety of ways. One common example of photochemistry is photosynthesis, the technique used by plants to generate energy from light, using the sun as a catalyst for chemical reactions which take place inside the plant. Bioluminescence, another natural phenomenon, is another excellent example of photochemistry at work; animals use bioluminescence for everything from signaling other organisms, as fireflies appear to do, to luring in prey, as seen with anglerfish in the ocean. Photography also relies on the principles of photochemistry, from the use of photosensitive compounds on film which react when exposed to light to the employment of stop and fixer in the developing room to put a halt to photosensitive reactions which occur in the coatings used on photo paper.
