Psychophysics
The science relating physical stimuli to the sensations they produce.
What it means
Psychophysics is the quantitative study of the relationship between physical properties of stimuli and the subjective sensations and perceptions they elicit. Founded by Fechner, it pioneered rigorous methods — limits, constant stimuli, and adjustment — for measuring absolute and difference thresholds and for scaling perceived magnitude. Its classical achievements include Weber's law, Fechner's logarithmic law, and Stevens' power law, while its modern form integrates signal detection theory to separate sensitivity from decision bias. By showing that inner experience could be measured against external reality, it helped make psychology an experimental science. It remains the methodological foundation of sensation and perception research.
Examples
Measuring exactly how much louder a tone must be to sound 'twice as loud' is a psychophysical question.
Dimmer switches are built so that turning the dial halfway looks halfway bright. Equal steps of electrical power would not look equal — measuring that gap between the physical and the felt is psychophysics.
Testing how much salt you must add before a soup tastes noticeably saltier, and finding the amount grows as the soup gets saltier, is Weber's law measured with a spoon.
First described in Gustav Fechner (1860); methods extended by S. S. Stevens.