Optical illusions trick the brain by exploiting the shortcuts it uses to interpret light and edges.

The longer version

The visual system does not measure the world; it guesses, using built-in assumptions about lighting, depth, and edges. Illusions are constructed cases where those assumptions are wrong, so the guess is confidently reported as perception even when the physical stimulus says otherwise.

Many illusions hinge on contrast, where a region is judged against its immediate surroundings rather than in absolute terms. Others exploit the brain's habit of assuming light comes from above, which determines whether a shape reads as a bump or a dent. Artists have used such tricks for centuries to suggest depth on flat surfaces.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.