Fraser spiral illusion

The Fraser spiral illusion is an optical illusion that was first described by the British psychologist Sir James Fraser (1863–1936) in 1908.^[1]

Fraser spiral illusion

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).^[2] 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.^[3] When the strands are highlighted in a different colour, it becomes obvious to the observer that no spiral is present.^[4]

See also

• Op-art
• Mathematics and art