Weber's Law
The just-noticeable difference between two stimuli is a constant proportion of the original stimulus intensity.
Also known as: Weber-Fechner Law
Category: Psychology & Mental Models
Tags: psychology, perception, psychophysics
Explanation
Weber's Law (1834) is one of the first quantitative laws in psychology, foundational to the field of psychophysics. It states that the just noticeable difference (JND) between two stimuli is a constant proportion of the original stimulus, expressed as ΔI/I = k, where ΔI is the change in stimulus, I is the original stimulus intensity, and k is a constant that varies by sensory modality.
For example, to notice a weight difference, you need about a 2% change regardless of whether you're lifting 1kg or 50kg. For brightness, the threshold is approximately 8%. This explains why we perceive relative rather than absolute differences in stimulation.
Gustav Fechner extended this into Fechner's Law, proposing that sensation grows logarithmically with stimulus intensity (S = k·log I). Stevens' Power Law later revised this relationship to a power function. Weber's Law has applications beyond perception, influencing our understanding of economics (diminishing marginal utility), UX design, and any domain where humans evaluate relative differences.
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