Sander Illusion

The Sander illusion, also called Sander's parallelogram, is a geometric optical illusion in which a diagonal line bisecting a larger parallelogram appears considerably longer than an identical diagonal line bisecting a smaller parallelogram placed beside it, even though the two diagonals are actually the same length. The German psychologist Friedrich Sander described the illusion in 1926, though the illusion researcher Matthew Luckiesh had already published essentially the same figure earlier, in his 1922 book Visual Illusions: Their Causes, Characteristics, and Applications. Two explanations are generally offered: the surrounding diagonal lines create an impression of depth that distorts the perceived length of the lines within it, and the acute angles present in the smaller parallelogram tend to make lines look shorter while the obtuse angles in the larger one tend to make lines look longer. The psychologist Wolfgang Metzger later extended this second account, observing that within non-right-angled parallelograms generally, a longer diagonal tends to look shortened while a shorter one tends to look lengthened.