# Weber's Law Weber's Law (1834) states that the just noticeable difference (JND) between two stimuli is a constant proportion of the original stimulus: ΔI/I = k. To notice a weight difference, you need about 2% change regardless of whether you're lifting 1kg or 50kg. This was one of the first quantitative laws in psychology, foundational to [[Psychophysics]]. [[Gustav Fechner]] extended this into Fechner's Law: sensation grows logarithmically with stimulus intensity (S = k·log I). [[Stevens' Power Law]] later revised this to a power function. Weber's Law explains why we perceive relative rather than absolute differences—a key principle in perception, economics (diminishing marginal utility), and UX design. ## The Law | Formula | Meaning | |---------|---------| | ΔI/I = k | JND is constant proportion | | k varies by modality | ~2% for weight, ~8% for brightness | ## References - https://en.wikipedia.org/wiki/Weber%E2%80%93Fechner_law ## Related - [[Psychophysics]] - [[Gustav Fechner]] - [[Stevens' Power Law]] - [[Perception]]