Photolysis refers to any chemical reaction in which a compound is broken down by light.
Photolysis is a part of photosynthesis, which occurs in the granum of the chloroplast. In photolysis the light absorbed by the chlorophyll is turned into chemical energy which is used to split water into hydrogen and oxygen. The oxygen is released as a byproduct while the hydrogen binds with the coenzyme NADP to form NADPH.
Photolysis is also a process in the atmosphere by which primary pollutants such as hydrocarbons and nitrogen dioxide are combined by sunlight to form secondary pollutants such as peroxyacyl nitrates. See photochemical smog.