 Square A is exactly the same shade of grey as square B. |
An optical illusion is any illusion that deceives the human visual system into perceiving something that is not present or incorrectly perceiving what is present. There are physiological illusions and cognitive illusions. A mirage is an example of a natural illusion that is an optical phenomenon, another example is the variation in the apparent size of the Moon (smaller when overhead, larger when near the horizon), this is not an optical phenomenon, but rather a cognitive or perceptual illusion.
Photo credit: Edward H. Adelson |