light: What are photo- chemical reactions?

  What are photo- chemical reactions?
Any reaction that takes place only in the presence of light is known as a photo-chemical reaction. The light may be visible light, infrared radiation, or ultra-violet radiation. Many photochemical reaction take place in nature, and the best example is photosynthesis. In photosynthesis, green plants make food using sunlight to combine carbon dioxide and water. Therefore, plants convert light energy into the chemical energy of food. Photochemical changes are also part of industrial processes, such as the formation of images in photog-raphy. When a picture is taken, some of the silver salts on the photographic film absorb light, and chemically change into metallic silver, which produces a dark image on the negative when the film is developed.

 We know that light is made of photons travelling at different wavelengths which make up different colours of the spectrum. But not all wavelengths will trigger a specific photochemical reaction. For example, a photon of violet light has highest frequency, and hence, the highest energy. Hence, a reaction which is initiated by violet light may not be initiated by red, blue, or other wavelengths of light. Another inter-esting fact is that some substances do not react directly when exposed to light, but the addition of another substance will trigger a photo-chemical reaction.