Fraser spiral illusion

{{short description|Optical illusion}}

Image:Fraser spiral.svg

The Fraser spiral illusion is an optical illusion that was first described by the British psychologist Sir James Fraser (1863–1936) in 1908.{{Cite journal |last=Fraser |first=James |date=January 1908 |title=A New Visual Illusion of Direction |url=https://bpspsychub.onlinelibrary.wiley.com/doi/10.1111/j.2044-8295.1908.tb00182.x |journal=British Journal of Psychology |language=en |volume=2 |issue=3 |pages=307–320 |doi=10.1111/j.2044-8295.1908.tb00182.x |issn=0950-5652}}

The illusion is also known as the false spiral, or by its original name, the twisted cord illusion. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.

The visual distortion is produced by combining a regular line pattern (the circles) with misaligned parts (the differently colored strands).{{cite book |last1=Cucker |first1=Felipe |author1-link=Felipe Cucker|title=Manifold Mirrors: The Crossing Paths of the Arts and Mathematics |date=2013 |publisher=Cambridge University Press |isbn=978-0-521-72876-8 |pages=163–166}} Zöllner's illusion and the café wall illusion are based on a similar principle, like many other visual effects, in which a sequence of tilted elements causes the eye to perceive phantom twists and deviations.

The illusion is augmented by the spiral components in the checkered background. It is a unique illusion, where the observer can verify the concentric strands manually.{{cite book |url=https://books.google.com/books?id=VfwhAQAAQBAJ&pg=PA57 |page=57 |title=Philosophy and Theatre: An Introduction |first1=Tom |last1=Stern |publisher=Routledge |date=October 1, 2013 |isbn=9781134575985}} When the strands are highlighted in a different colour, it becomes obvious to the observer that no spiral is present.See {{cite web |url=http://www.michaelbach.de/ot/ang_fraser/index.html |title=Fraser Illusion |work=Visual Phenomena & Optical Illusions |first1=Michael |last1=Bach |access-date=2005-06-11 |archive-date=2011-11-02 |archive-url=https://web.archive.org/web/20111102055445/http://www.michaelbach.de/ot/ang_fraser/index.html |url-status=dead }}