Optical illusions are images or pictures that we perceive differently than they really are. Put another way, optical illusions occur when our eyes send information to our brains that tricks us into perceiving something that does not match reality.
The Mach band illusion is an example of a physiological illusion. The line in the middle of the picture is one solid color. However, because of how the eye's retina filters the different shades on either side of the line, the right side of the line appears darker, while the left side of the line appears lighter.
Other optical illusions are cognitive. Cognitive illusions, such as ambiguous, distorting and paradox illusions, occur when our brains automatically make assumptions based on the information sent from the eyes. These illusions are sometimes called “mind games."
Doesn't the line in the middle look longer than the ones above and below it? However, all three lines are the same length!
Paradox illusions occur as a result of pictures or objects that cannot exist or are physically impossible. Paradox illusions are popular in works of art, such as those made famous by artist M. C. Escher.
Scientists believe optical illusions are possible because our brains are so good at recognizing patterns and “seeing" familiar objects. Our brains work quickly to make a “whole" image from separate pieces.
Clever artists can use these tendencies to trick our eyes and brains into seeing what's not really there!