Sander illusion

The Sander illusion or Sander's parallelogram is an <a href="/facts/Optical_illusion/8wfWwjFI">optical illusion</a> described by the German psychologist Friedrich Sander (1889–1971) in 1926. However, it had been published earlier by <a href="/facts/Matthew_Luckiesh/6AnGt3ub">Matthew Luckiesh</a> in his 1922 book <a href="http://www.openlibrary.org/details/visualillusionst00luckrich">Visual Illusions: Their Causes, Characteristics, and Applications</a> <a href="https://web.archive.org/web/20081121131711/http://www.openlibrary.org/details/visualillusionst00luckrich">Archived</a> 2008-11-21 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>.
The diagonal <a href="/facts/Line_(geometry)/gJqsr4Gj">line</a> bisecting the larger, left-hand <a href="/facts/Parallelogram/DRuktq60">parallelogram</a> appears to be considerably longer than the diagonal line bisecting the smaller, right-hand parallelogram, but it is 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.

Sander illusion open-in-new

Sander illusion