Stevens' Power Law is a principle in psychophysics that describes the relationship between the intensity of a stimulus and the perceived strength of that stimulus. Proposed by psychologist S.S. Stevens in 1957, it states that the perceived magnitude of a stimulus is proportional to its actual intensity raised to a specific power. This means that as the intensity of a stimulus increases, the perceived intensity does not increase linearly but rather follows a power function. The law applies to various sensory modalities, including vision, hearing, and touch. For example, in the case of brightness, a doubling of light intensity may not result in a perceived doubling of brightness. Instead, the perceived change depends on the exponent used in the power function, which varies for different types of stimuli.