TypeMotion
InteractiveYes: sliders and a check
First described2001
The yellow and blue bars move at exactly the same speed, but they seem to walk in turns.
Try this
- Tick Remove the stripes: they glide together.
- Change Speed.
Why it happens
Stuart Anstis described it in 2001. Contrast affects how fast things look: a bar looks faster when it has high contrast with the stripe behind it and slower when it has low contrast. The yellow bar has high contrast on black stripes and low contrast on white ones; the blue bar is the reverse. So as they cross the stripes, each seems to speed up and slow down at different moments.
Did you know?
Anstis also described "inchworms", where the front and back of one bar seem to move at different times.
References
- Anstis S. (2001). Footsteps and Inchworms: Illusions Show That Contrast Affects Apparent Speed. Perception, 30, 785-794. doi:10.1068/p3211
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Do the bars move at the same speed?
Yes. Remove the stripes to see.
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