Fraser Spiral Illusion

The Fraser spiral illusion, also called the false spiral or, by its original name, the twisted cord illusion, is an optical illusion first described by the British psychologist Sir James Fraser in 1908. Overlapping black arc segments in the image appear to form a continuous spiral, but the arcs are actually a series of concentric circles; the effect arises from combining a regular line pattern, the circles themselves, with misaligned parts, differently colored strands twisted together to make up each arc, similar in principle to the Zollner illusion and the Cafe wall illusion, where a sequence of tilted elements tricks the eye into perceiving phantom twists that are not really there. The checkered background pattern in the classic image further strengthens the spiral impression, and the illusion is unusual in that a viewer can manually trace a single concentric strand with a finger to directly verify that no true spiral exists; once the strand colors are changed, the illusion of a spiral disappears entirely.