An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, colour, size, position, tilt, movement), and cognitive illusions, the result of unconscious inferences...

See the complete article here: http://en.wikipedia.org/wiki/Optical_illusion

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You should be aware of the following illusions:

Visual illusions

Ponzo illusion
Ames room

moon illusion

Muller-Lyer illusion