fundamental principle relating to chemical reactions (chemical reaction) induced by light, which states that for every quantum of radiation that is absorbed, one molecule of the substance reacts. A quantum is a unit of electromagnetic radiation with energy equal to the product of a constant ( Planck's constant, h) and the frequency of the radiation, symbolized by the Greek letter nu (ν). In chemistry, the quantitative measure of substances is expressed in terms of gram moles (mole), one gram mole comprising 6.02214179 × 10²³ ( Avogadro's number) molecules. Thus, the photochemical equivalence law is restated as: for every mole of a substance that reacts 6.02214179 × 10²³ quanta of light are absorbed.

The photochemical equivalence law applies to the part of a light-induced reaction that is referred to as the primary process; that is, the initial chemical change that results directly from the absorption of light. In most photochemical reactions (photochemical reaction) the primary process is usually followed by so-called secondary processes that are normal interactions between reactants not requiring absorption of light. As a result such reactions do not appear to obey the one quantum–one molecule reactant relationship. The law is further restricted to conventional photochemical processes using light sources with moderate intensities; high-intensity light sources such as those used in flash photolysis and in laser experiments are known to cause so-called biphotonic processes; i.e., the absorption by a molecule of a substance of two photons (photon) of light.

The photochemical equivalence law is also sometimes called the Stark–Einstein law after the German-born physicists Johannes Stark (Stark, Johannes) and Albert Einstein (Einstein, Albert), who independently formulated the law between 1908 and 1913.