Optical illusions come in many forms, and just when we think we've seen all of the best ones and become immune to their effects, along comes another mind bender to confound us even more. The optical illusion below shows two rectangles, one blue and one yellow, moving across a black and white striped background.
The two rectangles appear to be in a race with each other, but it's neck and neck: no sooner does one pull ahead, then the other catches up. But actually it turns out that the two rectangles are moving together at exactly the same pace. And there's another level to the confusion when the background changes (for more, see our pick of the best must-see optical illusions).
Reality shatter. The two objects are traveling in exactly the same manner. Watch when it turns gray. https://t.co/gkRKyfNnyI pic.twitter.com/HpZsudeXSGNovember 26, 2017
Shared by optical illusion fan Cliff Pickover on Twitter a few years back, this baffling non-race is still melting people's minds online today.
So what's going on? Well, the impression that they're advancing at a different rate is caused by the striped background. Just to confuse us more, when the stripes are replaced with plain grey, the racing rectangles appear to slow down slightly, although they're actually still moving at the same speed.
Known as a stepping feet optical illusion, the phenomenon has been explored in the academic paper Footsteps and Inchworms by Stuart Anstis of the University of California. The research describes how a stationary surround, such as the striped background in the gif above, can "profoundly affect our perception of moving objects". Low contrast reduces how fast an object appears to move. The paper noted that as a result of this moving cars can look like they're slowing down in foggy conditions.
What's happening in the optical illusion above is that when the blue rectangle moves into a new white stripe, the high contrast between the blue and white makes it more visible and so it seems to moving faster than the yellow rectangle. Meanwhile, the yellow rectangle creates the same optical illusion of speed when entering a new black stripe, again because of the contrast. When the stripes are replaced with a grey background, the difference in contrast is neutralised and we can see they are actually moving at the same pace. Yes, my head hurts too.
It seems optical illusions will continue to blow our minds, even ones that have already been explored in such detail. For more, see our pick of the most mind-blowing optical illusions of 2022 and the best animal optical illusions. If you're looking to make your own, see our pick of the best graphic design software, or see the best current prices on Adobe's Creative Cloud suite below.