Sander illusion
{{Short description|Optical illusion}}
{{one source |date=April 2024}}
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] {{Webarchive|url=https://web.archive.org/web/20081121131711/http://www.openlibrary.org/details/visualillusionst00luckrich |date=2008-11-21 }}.{{cite book |chapter=Sander parallelogram n. |year=2006 |title=A Dictionary of Psychology |first=Andrew M. |last=Colman |publisher=Oxford University Press}}
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 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.