Jot down some important points about optical illusion from the given passage.
In��visual perception, an��optical illusion��(also called a��visual illusion ) 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��but a
 classification 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. 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).��An example for a
 physiological fiction is an��afterimage. Three typical cognitive distortions are the�� Ponzo ,��Poggendorff,
 and��M��ller-Lyer��illusion. Physical illusions are caused by the physical environment, e.g. by the optical
 properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of
 excessive stimulation of a specific receptor type. ��Cognitive��visual illusions are the result of��unconscious
 inferences��and are perhaps those most widely known.
1. Visual Illusion is caused by the��visual system��and characterized by a visual��percept��that appears
 to be different from��reality.
 2. According to Richard Gregory a classification proposed by��him is useful as an orientation that,
 there are three main classes: physical, physiological, and cognitive illusions, and in each class
 there are four kinds: Ambiguities, distortions, paradoxes, and fictions.
 3. An 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
 5. Three cognitive distortions are the�� Ponzo ,��Poggendorff, and��M��ller-Lyer��illusion.
 6. Physical illusions are caused by the physical environment, e.g. by the optical properties of
 water. Physiological illusions arise in the eye or the visual pathway.
 7. The most widely know illusion is visual illusions which are the result of��unconscious inferences.