• noun the study of chemical changes brought about by light and other forms of radiation
  • The study of chemical reactions occurring under the influence of incident light. Examples of astronomical interest include the Earth’s upper atmosphere, where sunlight catalyses the production of ozone from conventional oxygen molecules, and that of Titan, where molecules including ethylene, acetylene, hydrogen cyanide and others are produced by photochemical reactions involving the nitrogen and methane which make up most of Titan’s atmosphere.

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