For photochemical reactions that are reversible, such as certain photocycloadditions or photoisomerisations, a wavelength dependence in photoproduct yield emerges due to competing forward and backward reactions and overlapping absorbance of the reactant and product. Such an effect can contribute to the mismatch between maximum reactivity and absorbance. Under simple conditions, the effect can be mathematically modelled by solving differential rate equations to determine a solution for the most effective wavelength for maximising product yield.