Müller-Lyer illusion
Two equal lines look unequal because of the arrowheads on their ends.
What it means
The Müller-Lyer illusion is a classic visual illusion in which two lines of identical length appear different because one ends in outward-pointing fins and the other in inward-pointing ones, with the outward-fin line looking longer. A leading explanation holds that the fins act as depth cues reminiscent of inside versus outside corners, so the visual system misapplies size constancy and rescales the lines. Strikingly, the illusion's strength varies across cultures, being weaker among people raised in environments without carpentered, right-angled structures, which supports the role of learned perceptual inference. It demonstrates that even simple length judgments are shaped by top-down assumptions about three-dimensional scenes. It is a staple example of perception as construction rather than measurement.
Examples
Asked which shaft is longer, observers reliably pick the one with outward-spreading fins, though both are exactly equal.
Lay a ruler across both shafts and the illusion does not budge. You can know they are equal, measure them equal, and still see them as different lengths.
Rearranging a room, a shelf ending at an outward-jutting corner looks longer than an identical shelf tucked into an alcove, so the pair never quite seems to line up.
First described in Franz Carl Müller-Lyer (1889).