The red square seems to bow, its sides curving and its corners pulled out of true. It is a perfect square.
Try this
- Change Number of circles: does a busier background make the bend stronger?
- Tick Hide the circles and the square snaps back to straight.
Why it happens
The circles behind the square change how your visual system reads the angles where the square's edges cross them. Each crossing is a small angle, and the brain tends to exaggerate small acute angles, so the straight edge seems to tilt a little at every crossing. Added up along the edge, those tiny tilts read as a curve. William Orbison described a whole family of these figures in 1939 and tried to explain them as distortions caused by a "field" set up by the background lines. Later work, reviewed by Gerald Westheimer, treats them as part of the same group as the Hering and Zöllner illusions: line orientations being pushed apart by the lines around them.
Did you know?
Orbison drew squares, circles and triangles over circles and radiating lines, and found that the same shape bends in different directions depending on the pattern behind it.
References
- Orbison W. (1939). Shape as a Function of the Vector-Field. The American Journal of Psychology, 52, 31-45. doi:10.2307/1416658
- Westheimer G. (2008). Illusions in the spatial sense of the eye: Geometrical-optical illusions and the neural representation of space. Vision Research, 48, 2128-2142. doi:10.1016/j.visres.2008.05.016
- 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 the square really straight?
Yes. Hide the circles to check.
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