Stark–Einstein Law


he Stark–Einstein law is named after German-born physicists Johannes Stark and Albert Einstein, who independently formulated the law between 1908 and 1913. It is also known as the photochemical equivalence law or photoequivalence law. In essence it says that every photon that is absorbed will cause a (primary) chemical or physical reaction.[13]

The photon is a quantum of radiation, or one unit of radiation. Therefore, this is a single unit of EM radiation that is equal to Planck’s constant (h) times the frequency of light. This quantity is symbolized by γ, hν, or ħω.

The photochemical equivalence law is also restated as follows: for every mole of a substance that reacts, an equivalent mole of quanta of light are absorbed. The formula is shown here.

Related formulas


ΔEmolenergy absorbed by each reacting atom or molecule (J/mol)
NAAvogadro constant
hPlanck constant
cSpeed of light
λwavelength (m)