An optical illusion is when images are placed in a certain position which causes the image to be misleading. The information that is then collected by the eye is processed by the brain and makes the brain think that it is seeing something completely different. A physiological illusion is where the effects on the eyes and brain of excessive stimulation of a specific type (brightness, the tilt, the color, movement). A cognitive illusion is when the eye and brain make unconscious conclusions from evidence or reasoning. Color optical illusions are caused by the different colors being placed next to each other. Many optical illusions are errors of sense, judgment, defects of the optical system, and many are caused my certain characteristics of the visual process.Simultaneous Contrast is when one color is placed on two separate colors that are placed next to each other. The one color then appears to be two separate colors due to brightness and contrast. This can also be done using shades.
After images are when someone stares at brightly colored objects for 30 seconds and then quickly switches to a white surface where the inverse of the original image is shown. This is caused by fatigue of the visual process. Therefore after images are best seen when eyes are well rested.
However these are just a few examples of optical illusions. Optical illusions can be caused by several different conditions, whether it be placement, color, shades, and the viewers specific eye.
http://www.colorcube.com/illusions/scstripe.htm
http://www.colorcube.com/illusions/optiart.htm http://www.visualillusion.net/Chap09/Page01.php
http://en.wikipedia.org/wiki/Optical_illusions
