(the reciprocity law), one of the fundamental laws of photochemistry; discovered by R. Bunsen and the English chemist H. Roscoe in 1862. According to this law, the amount of product of a photochemical reaction is determined by the total amount of radiant energy falling on the photochemical system—that is, the product of the radiation intensity Φ and the exposure time t—without regard for the dependence on the relation of the cofactors Φ and t.

The Bunsen-Roscoe law is observed in those cases in which the primary photochemical reaction is not accompanied by secondary reactions of other types and is not complicated by the inhibiting action of accompanying substances, including the reaction products themselves. However, in many cases—in a broad range of ratios of Φ and t—this law is not observed. The nonobservance of this law in photography is of practical value. In modern silver-halide films, the Bunsen-Roscoe law is usually observed only for exposures in two ranges: from 0.1 to 0.01 sec, and less than 10⁻⁵ sec. In the process of vision, the Bunsen-Roscoe law is observed for light pulses of 0.01 through 0.001 sec duration.