TypeLight and color
InteractiveYes: sliders and a check
First described1970
The left half looks lighter than the right, but apart from the edge, both are the same gray.
Try this
- Tick Cover the edge and the halves match.
- Set Strength of the edge to 0: no edge, no difference.
Why it happens
Your brain judges the brightness of large areas mostly from their edges, then fills in the inside. A sharp step at the edge says "light on this side, dark on that side", and the brain spreads that judgment across each half. Dale Purves, Amita Shimpi and Beau Lotto argued that this matches how often such edges come from real differences in surface brightness in natural scenes.
Did you know?
Tom Cornsweet described it in his 1970 book Visual Perception; similar effects are also called Craik-O'Brien-Cornsweet.
References
- Purves D., Shimpi A., Lotto R. (1999). An Empirical Explanation of the Cornsweet Effect. The Journal of Neuroscience, 19, 8542-8551. doi:10.1523/jneurosci.19-19-08542.1999
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Are both halves of the Cornsweet illusion the same?
Yes, except at the thin edge. Cover the edge to check.
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