The rate of a photochemical reaction is directly proportional to the intensity of light. This relationship is described by the Bunsen-Roscoe law, also known as the reciprocity law. According to this law, the product of the light intensity I and the exposure time t is a constant k for a given photochemical reaction:I t = kAs the intensity of light increases, the rate of the photochemical reaction also increases, leading to a faster reaction. Conversely, as the intensity of light decreases, the rate of the photochemical reaction decreases, resulting in a slower reaction.This relationship can be explained by considering that photochemical reactions involve the absorption of light by molecules, which leads to the formation of excited states. These excited states can then undergo various processes, such as dissociation, isomerization, or electron transfer, to produce the products of the reaction. The higher the light intensity, the more photons are available to be absorbed by the molecules, leading to a higher probability of forming excited states and, consequently, a faster reaction rate. On the other hand, when the light intensity is lower, fewer photons are available for absorption, resulting in a lower probability of forming excited states and a slower reaction rate.