Sander illusion

The Sander illusion or Sander's parallelogram is an optical illusion described by the German psychologist Friedrich Sander (1889-1971) in 1926. However, it had been published earlier by Matthew Luckiesh in his 1922 book " [http://www.openlibrary.org/details/visualillusionst00luckrich Visual Illusions: Their Causes, Characteristics, and Applications] " [Sander parallelogram n. (2006). A Dictionary of Psychology. Andrew M. Colman. Oxford University Press, 2006.] .

The diagonal line bisecting the larger, left-hand parallelogram appears to be considerably longer than the diagonal line bisecting the smaller, right-hand parallelogram, but is in fact the same length.

One possible reason for this illusion is that the diagonal lines around the blue lines give a perception of depth, and when the blue lines are included in that depth, they are perceived as different lengths.

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