TheSander illusion orSander's parallelogram is anoptical illusion described by the German psychologist Friedrich Sander (1889–1971) in 1926. However, it had been published earlier byMatthew Luckiesh in his 1922 bookVisual Illusions: Their Causes, Characteristics, and ApplicationsArchived 2008-11-21 at theWayback Machine.[1]
The diagonalline bisecting the larger, left-handparallelogram 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.
Another explanation is that the illusion is caused by theacute angles in the right parallelogram and theobtuse angles in the left parallelogram, which deceptively influence the perception of line lengths. Obtuse angles have a strong tendency to seemingly lengthen the lines involved. The left diagonal divides two obtuse interior angles and therefore appears longer, while the right diagonal divides two acute interior angles and is therefore perceived as shortened.
The GermanpsychologistWolfgang Metzger (1899–1979) also described the generalphenomenon in this context that in a (non-right-angled) parallelogram, the longer diagonal appears shortened and the shorter one appears longer.[2]
