Fechner’s Law

Table of Contents

Read here about a physical phenomenon that is exponential in nature.


Fechner’s law, which states that the perceived magnitude (S ) is a logarithmic function of stimulus intensity (I ) multiplied by a modality and dimension pecific constant (k).


Ernst Weber discovered what we now know as Weber’s Law, that the discriminability of two magnitudes (two sound or light intensities) is a function of their ratio: over a large dynamic range, and for many parameters, the threshold of discrimination between two stimuli increases linearly with stimulus intensity.

Gustav Theodor Fechner of Leipzig soon afterwards saw an intrinsically logarithmic component in psychophysics, or so he realized on 22 October 1850 while still in bed. (Information: the new langauge of science, By Hans Christian Von Baeyer, Harvard university Press, 2004). He realized that Weber’s law could be accounted for by postulating that the external stimulus is scaled into a logarithmic internal representation of sensation. In Fechner’s law the magnitude of a sensation is proportional to the logarithm of the intensity of the stimulus, measured as a multiple of the smallest perceptible stimulus. The latter condition removes the units from the expression, making is a general rule. The “law” held sway until the 1950’s where deviations outnumbered conformances.