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 Müller-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.
Jot down some important points about optical illusion from the given passage.
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 Müller-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.