An visual optical illusion is characterized through a visual image that is perceived different from the real object. The information gathered by eye is processed by the brain so that it can give us a perception on the reality, but it does not involve any physical measurement.
What to see?
In this image there is a green cylinder standing on a board made out of gray and white squares, a check board. Edward Adelson made this illusion by labeling two squares A and B. A appears to be a darker color than B, even if they actually have the same color.
How it works?
This image is a three dimensional one and this is the reason why the colors appear different to us.
Because our visual system tries to interpret a 3-dimensional scene it estimates lighting vector and uses it to analyze the properties of the material. If we analyze the amount of light that is on a surface it will not be enough because our visual system uses some tricks to determinate where are the shadow and how to compensate for them, in order to determinate the kind of gray that belong to the checker board.
So if we think logical: even if a square is lighter than the ones around it’s probably lighter than average and if a square is darker than the ones around it than it’s probably darker than average. So even though the square is physically dark, it is light when compared to the squares around it. The dark squares outside the shadow are surrounded by lighter checks, so they look dark by comparison.
Another trick is given by the shadow. Our visual system tends to ignore gradual changes in the light level, so that it can determine the color of the surfaces without being misled by shadows.