Ebbinghaus Illusion

The Ebbinghaus illusion was discovered by the German psychologist Hermann Ebbinghaus in the late 1800: the most famous version of this illusion is a couple of orange circles, one surrounded by large grey circles, the other by smaller grey circles. The result is a size perception illusion, where one orange circle looks smaller than the other, while they are in effect the same size.

Ebbinghaus illusion: the two orange circles are exactly the same size, but they seem different

Ebbinghaus illusion on a blue background