Cornsweet Illusion

The Craik-O'Brien-Cornsweet Illusion is a color/brightness perception illusion, observed first in 1960 by Tom Cornsweet.

In this illusion, the right half of the image looks brighter than the left, but they are really the same color and brightness. This effect is similar to those obseved in other contrast experiments (such as the Mach bands) but it presents important differences, namely the fact that a small stripe alters the perception of a (relatively) much larger area.

Another important remarkable detail is that the area immediately adjacent to the dark line is brighter in the right half, and darker in the left half, whereas other contrast experiments usually show the opposite phenomenon.

Craik-O'Brien-Cornsweet Illusion: the two squares look different, but their colors and brightness are exactly the same (see the next picture)

The illusion is revealed when we enlarge the central divisor: you can now see the colors are the same