Weber's Law

The Weber–Fechner law attempts to describe the relationship between the physical magnitudes of stimuli and the perceived intensity of the stimuli. Ernst Heinrich Weber (1795–1878) was one of the first people to approach the study of the human response to a physical stimulus in a quantitative fashion. Gustav Theodor Fechner (1801–1887) later offered an elaborate theoretical interpretation of Weber's findings, which he called simply Weber's law, though his admirers made the law's name a hyphenate.
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Noun
1. (psychophysics) the concept that a just-noticeable difference in a stimulus is proportional to the magnitude of the original stimulus; "Weber's law explains why you don't notice your headlights are on in the daytime"
(hypernym) law, law of nature
(classification) psychophysics

to double the effect of an original stimuli (seeing, smelling, hearing) the stimuli must be increased by more than 100% e.g. for a newspaper ad to be perceived as double in size by the audience it must be increased by 4 times the original size; get it??