The rate of a photochemical reaction involving a certain reactant 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 rate of a photochemical reaction is directly proportional to the product of the intensity of light I and the exposure time t . Mathematically, it can be expressed as:Rate I tWhen the intensity of light is increased, the rate of the photochemical reaction also increases. Conversely, when the intensity of light is decreased, the rate of the photochemical reaction decreases. This is because the number of photons light particles interacting with the reactant molecules is higher when the light intensity is higher, leading to more frequent and effective collisions that result in the formation of products.It is important to note that the relationship between the rate of a photochemical reaction and the intensity of light may not always be linear, especially at very high or very low light intensities. At very high intensities, the reactant molecules may become saturated with photons, and the rate may not increase as rapidly. At very low intensities, the rate may be too slow to measure accurately.