photochemical reaction

Any chemical reaction caused by absorption of light (including visible, ultraviolet, and infrared). The light excites atoms and molecules (shifts some of their electrons to a higher energy level) and thus makes them more reactive. In comparison to ordinary reactions using thermal energy alone, photochemical reactions can follow different routes and are more likely to produce free radicals, which can trigger and sustain chain reactions.

This type of reaction is seen in the bleaching of dyes or the yellowing of paper by sunlight. It is harnessed by plants in photosynthesis and by humans in photography.

One photochemical reaction is the action of sunlight on car exhaust fumes, which results in the production of ozone. Some large cities, such as Los Angeles, USA, and Santiago, Chile, now suffer serious pollution due to photochemical smog.