Law of photochemical equivalence represents a basic law in photochemistry. Quantum theory of light states that the energy is emitted or absorbed in small groups of quanta, having the energy `e = h*nu` .
"`nu` " represents the light frequency
'`h` " represents the Planck's constant
The photochemical equivalence law was discovered by Einstein, in 1905, when he applied quantum theory of light to photochemical reactions. The photochemical equivalence law stated that an atom or molecule absorbs one quantum only, if it absorbs light having a given frequency.
The fact that only one photon is absorbed by molecule can be explained by the circumstance of only one molecule being activated by the quantum of radiation. If there is more than one molecule activated, then more than one photons are absorbed.