Both purple disks are identical. The one wrapped in a close ring looks larger.
Try this
- Slide Ring around the left disk from tight to wide and watch the disk seem to shrink.
- Tick Hide the ring to compare.
Why it happens
Like the Ebbinghaus illusion, this is a contrast and blending effect. When the ring is close, your brain seems to fold the ring into the size of the disk, so it reads bigger. When the ring is far away, the empty space makes the disk look small by comparison.
The same effect shows up on dinner plates: a portion of food looks smaller on a big plate, a finding some nutrition researchers have tested with mixed results.
Did you know?
Belgian philosopher Joseph Delboeuf described it in the 1860s, as part of early experiments on how we measure what we see.
References
- Nicolas S. (1995). Joseph Delboeuf on Visual Illusions: A Historical Sketch. The American Journal of Psychology, 108, 563. doi:10.2307/1423073
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Is it why food looks smaller on big plates?
That is the idea some researchers have tested, with mixed results.
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