A law in photochemistry stating that only the light absorbed by a substance or substances is effective in bringing about chemical change. Not all the light falling on the substances will necessarily bring about chemical change, since some of it can be re-emitted in the form of heat or light. The light does not need to be absorbed directly by the reacting substances; it is possible, in photosensitization for example, that light can be absorbed by an inert substance, which subsequently transfers the absorbed energy (as thermal energy) to the reactants. This law was discovered by Theodor Grotthuss (1785–1822) in 1817 and John William Draper (1811–82) in 1842.