An Optical Illusion is caused by info received by our eyes then wrongfully interpreted by the brain. The main classes of illusions are Physiological and Cognitive.
Physiological illusions are caused by excessive stimulation of brightness, color, contrast, movement, etc. Examples: Afterimages such as seeing "spots" after getting a camera flashed in your eyes.
This site is more about cognitive illusions than physiological ones. Cognitive illusions interact with our natural visual assumptions and are misinterpreted. Cognitive illusions are can be divided into ambiguous, distortion, and paradox illusions.
Ambiguous Illusions are images that 'change' appearance right before our eyes. Perception switches back and forth -- as available visual data does not confirm a single view. Illusions that give the impression of motion also fall into this group.
Distorting Illusions produce distortions of scale, size, or curvature.
Paradox illusions include images that are contradictory or not possible. These illusions are caused by our brain's misunderstanding that adjacent edges must join.
Click one of the above thumbnails to view more images from our optical illusion gallery.
Books about Optical Illusions and Visual Phenomenon.
Links to learn more about Optical Illusions.