The Delboeuf Illusion is an optical illusion that demonstrates how our perception of size can be altered by the context in which it is presented. It is based on the idea that when two circles of equal size are placed near each other, the one that is surrounded by a larger circle
The Delboeuf illusion is an optical illusion of relative size perception: In the best-known version of the illusion, two discs of identical size have been placed near to each other and one is surrounded by a ring; the surrounded disc then appears larger than the non-surrounded disc if the ring is close, while appearing smaller than the non-surrounded disc if the ring is distant. A 2005 study suggests it is caused by the same visual processes that cause the Ebbinghaus illusion.