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.
Square A is exactly the same shade of grey as square B.
This is possibly one of the most uncanny optical illusions in existence. No matter how hard you look, you may not be able to convince yourself that the two squares are the identical shade of grey.
Don’t believe it? Try this:
Right click the large picture, save it to your hard drive, put it into your image editor, “select” square A and then pull it down onto square B. There will be an exact match in color.