Absorption of radiation by reactants of a reaction at equilibrium increases the rate of forward reaction without directly affecting the rate of the reverse reaction.
The rate of a photochemical reaction is proportional to the absorption cross section of the reactant with respect to the excitation source (σ), the quantum yield of reaction (Φ), and the intensity of the irradiation. In a reversible photochemical reaction between compounds A and B, there will therefore be a "forwards" reaction of   at a rate proportional to   and a "backwards" reaction of  at a rate proportional to . The ratio of the rates of the forward and backwards reactions determines where the equilibrium lies, and thus the photostationary state is found at:
If (as is always the case to some extent) the compounds A and B have different absorption spectra, then there may exist wavelengths of light where σ is high and σ is low. Irradiation at these wavelengths will provide photostationary states that contain mostly B. Likewise, wavelengths that give photostationary states of predominantly A may exist. This is particularly likely in compounds such as some photochromics, where A and B have entirely different absorption bands. Compounds that may be readily switched in this way find utility in devices such as molecular switches and optical data storage.