The eye merely transmits light signals to the brain which then interprets them through a series of complex operations. It may misinterpret them, in which case an optical illusion occurs. Such illusions may, like mirages, originate naturally or be induced artificially by causing what appears to be a distortion on a test element, for example by drawing arrows on straight parallel lines.
Optical illusions reveal some of the ways our brain functions.
Click on an illusion to enlarge it, then drag the slider to reveal it.