Delboeuf illusion was named after Joseph Remi Leopold Delboeuf, a Belgian experimental psychologist, hypnotist, philosopher, and mathematician.
This optical illusion is exhibited by two identical discs being perceived differently due to relative size perception. The disc surrounded by a bigger circle (making the margins wider) seems smaller as compared to its identical counterpart which was surrounded by a smaller circle (making the margins narrower). One of the famous applications of this illusion is the advice to eat on smaller plates as it may give you the impression that you are eating a lot since your plate looks full. On the other hand, you may think that you are not eating that much when the same amount of food is placed on a bigger plate.