Grotthus-Draper law

Grotthus-Draper law radiation should be absorbed by the substance to perform a chemical change [199,200],... 

The processes of photochemistry are the same for polymers and small molecules. The Grotthus-Draper law sfafes fhat no photochemical reactions can occur unless a photon of lighf is absorbed. This means, for example, thaf many commercial plastics transparent in the near UV can undergo photodegradation only as a result of the absorption of light by impurities. 

The first law of photochemistry states Only that light which is absorbed by a system can cause chemipal change (Grotthus-Draper Law). 

When a photon passes close to a molecule, there is an interaction between the electromagnetic field associated with the molecule and that associated with the radiation. If, and only if, the radiation is absorbed by the molecule as a result of this interaction, can the radiation be effective in producing photochemical changes (Grotthus-Draper law, see, e.g., Finlay son-Pitts and Pitts, 1986). Therefore, the first thing we need to be concerned about is the probability with which a given compound absorbs uv and visible light. This information is contained in the compounds uv/vis absorption spectrum, which is often readily available or can be easily measured with a spectrophotometer. 

The Grotthus-Draper law states that photophysical/photochemical reactions occur only when a photon of light is absorbed. This forms the basis for the First Law of Photochemistry—that is, only light that is absorbed can have a photophysical/photochemical effect. 

This is an axiomatic principle of photochemical phenomena and is referred to as the Grotthus-Draper law. 

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