The Sander illusion or Sander's parallelogram is an
optical illusion
In visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide varie ...
described by the German psychologist Friedrich Sander (1889–1971) in 1926. However, it had been published earlier by
Matthew Luckiesh in his 1922 book
Visual Illusions: Their Causes, Characteristics, and Applications''.
The diagonal
line bisecting the larger, left-hand
parallelogram
In Euclidean geometry, a parallelogram is a simple polygon, simple (non-list of self-intersecting polygons, self-intersecting) quadrilateral with two pairs of Parallel (geometry), parallel sides. The opposite or facing sides of a 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.
References
Optical illusions
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