Psychology -> Muller-Lyer illusion

Muller-Lyer illusion

ANIMATION

This consists of two lines, one with 'arrow-heads' each end and the other, with 'fishtails' at each end. Although both lines are of equal length, we perceive the line with the 'fishtails' as being longer.

Richard Gregory explains this illusion in terms of depth cues and size constancy. He believes that we liken the line with 'fishtails' as being the inside corner of a room and the other line as the edge of a building's outer wall. Both lines cast the same size image on the retina however, as the line with the outward pointing arrowheads is perceived to be further away, we apply size constancy and interpret that it must be longer.