TypeSize and shape
InteractiveYes: sliders and a check
First described1855
The left span, filled with ticks, looks longer than the empty span on the right. They are exactly the same width.
Try this
- Change Number of ticks and see when the left span looks longest.
- Tick Show guide lines to see they match.
Why it happens
Divided spaces look bigger than empty ones. Your visual system seems to judge extent partly by how much is going on inside it. Jiří Wackermann and Kristina Kastner measured how the strength of the effect depends on how the space is filled, which is why it changes as you move the slider.
Did you know?
Johann Joseph Oppel described it in 1855 and Augustus Kundt studied it in 1863; it is also called the filled/empty space illusion.
References
- Wackermann J., Kastner K. (2010). Determinants of filled/empty optical illusion: Search for the locus of maximal effect. Acta Neurobiologiae Experimentalis, 70, 423-434. doi:10.55782/ane-2010-1814
- 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 the two spans the same width?
Yes, 320 pixels each. Show the guide lines to check.
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