Weber's Law and Sensory Adaptation

Weber's Law describes the just noticeable difference, or JND: the smallest change in a stimulus you can detect. The key claim is that the JND is not a fixed quantity, it's a constant fraction of the original stimulus. That fraction, called Weber's fraction, differs by sense (sound, weight, light, taste all have their own), but within a given sense it stays steady across most of the range. The practical result: the bigger the starting stimulus, the bigger the absolute change needed before you notice anything happened.

This explains why adding one candle to a dark room looks dramatic but adding one candle to a stadium full of lights looks like nothing. The absolute change is identical, one candle, but the proportion relative to the starting brightness is wildly different, so the JND is crossed in the dark room and not in the stadium.

Sensory adaptation sits right next to this idea because both describe how sensitivity depends on context rather than on the raw stimulus alone. Adaptation is what happens when a stimulus stays constant over time: receptors respond less and less, and the sensation fades, which is why you stop smelling a room you've been sitting in for ten minutes or stop feeling your watch on your wrist. Weber's Law is about detecting change across intensity, adaptation is about losing sensitivity to sameness over time, but together they show that perception tracks relative difference, not absolute amount.