What is Photochemistry?
[[File:Simple photosynthesis overview.svg|thumb|right|A scheme showing examples of photochemical reactions in nature]] '''Photochemistry''' is the study of [[chemical reaction]]s with light. There are many natural examples. One is [[photosynthesis]], how plants make [[oxygen]] from [[carbon dioxide]] and [[water]]. Another is making [[vitamin D]] in the body. Many times reactions go with heat: as you heat [[molecule]]s, they start to move around and so hit other molecules which can then react. What molecules need is just some [[activation energy]], a little energy to start the reaction. In the case of photochemistry, this energy is given by the light. A molecule or an [[atom]] absorbs at a certain [[wavelength]]. If light is given at this wavelength then the molecule or atom becomes excited. It is then easier for it to react. Another way in which light can help reactions go is by changing the [[symmetry]] of a molecule. This can be tested with the [[Woodward–Hoffmann rules]]. Some [[pericyclic reaction]]s go with heat, some go only with light. In [[atmospheric chemistry]], light is very important. This is because there is much more [[Ultraviolet|UV light]] at the top of the [[atmosphere]]. This allows reactions that do not happen on the ground. For example, [[ozone]] can split in an [[oxygen]] molecule and one oxygen atom. {{BranchesofChemistry}} {{Chemistry-stub}} {{authority control}} [[Category:Branches of chemistry]]