Optical Illusions


Focus on the spinning centre. After two cycles try to read a sentence in the the middle of the block of text below to experience the reverse rotation effect from the static text.

Within visual perception, an optical illusion (also called a visual illusion[2]) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear[3] but a classification[1][4] proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions.[4] A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect (where, despite movement, position remains unchanged).[4] An example for a physiological fiction is an afterimage.[4] Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion.[4] Physical illusions are caused by the physical environment, e.g. by the optical properties of water.[4] Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type.[4] Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.[4]