The leaning tower illusion is a

visual illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; thei ...

seen in a pair of identical images of the

Leaning Tower of Pisa The Leaning Tower of Pisa ( it, torre pendente di Pisa), or simply, the Tower of Pisa (''torre di Pisa'' ), is the ''bell tower, campanile'', or freestanding bell tower, of Pisa Cathedral. It is known for its nearly four-degree lean, the result ...

photographed from below. Although the images are duplicates, one has the impression that the tower on the right leans more, as if photographed from a different angle. The illusion was discovered by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu at

McGill University McGill University (french: link=no, Université McGill) is an English-language public research university located in Montreal, Quebec, Canada. Founded in 1821 by royal charter granted by King George IV,Frost, Stanley Brice. ''McGill Universit ...

, and won first prize in the

Best Illusion of the Year Contest The Best Illusion of the Year Contest is an annual recognition of the world's illusion creators awarded by the Neural Correlate Society. The contest was created in 2005 by professors Susana Martinez-Conde and Stephen Macknik as part of the Euro ...

2007.Best Visual Illusion of the Year contest 2007
/ref> The authors suggest that the illusion occurs because of the way the visual system takes into account perspective.The Leaning Tower illusion: a new illusion of perspective (Perception)
/ref>Leaning Tower Illusion (Scholarpedia)
/ref> When two identical towers rise in parallel but are viewed from below, their corresponding outlines converge in the retinal image due to perspective. The visual system normally "corrects" for the perspective distortion and as a result perceives the towers correctly, i.e. as rising in parallel. However in the case of the two identical images of the Pisa tower, the corresponding outlines of the towers do not converge but run in parallel, and as a result the towers are perceived as non-parallel, i.e. as diverging. The illusion reveals that the visual system is obliged to treat the two images as part of the same scene, in other words as the "Twin Towers of Pisa". Although the Pisa tower demonstrates the illusion and provides a pun for its name, the illusion can be seen in any pair of (identical) images of a receding object.

References

Optical illusions {{Psychology-stub