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How Optical Illusions Work

        Science | Optics

Now You See It, Now You Don't
In the Müller-Lyer illusion, the lines appear to be of differing lengths (because of the direction of the arrows), even though they are all the same.
In the Müller-Lyer illusion, the lines appear to be of differing lengths (because of the direction of the arrows), even though they are all the same.
Wikimedia Commons

Optical illusions are everywhere. Aristotle noticed optical illusions in waterfalls. Indiana Jones saw one in the rocks as he made his leap of faith across the giant crevasse in the "Indiana Jones and the Last Crusade". And we see them everywhere, from M.C. Escher drawings to trending internet memes (was that dress blue or gold?).

In fact, once we've seen the "trick" in an illusion, it's nearly impossible to un-see it. We simply can't transport our minds back to a time when we didn't know the thing we just learned. Once the prior knowledge is available, our brain quickly accesses it and puts it together with the visual cues that you have gotten from actually looking at the illusion. The "can't un-see" feeling that many people have when looking at illusions is a perfect example of the brain doing more than just translating what our eyes see.

And optical illusions are not just a function of our eyes and brains; our perception may also largely be influenced by cultural factors. While the biological basis for how optical illusions might work is universal across humans, when some illusions are shown to people in different cultures, not everyone saw the same thing or missed the same visual cues [sources: Schultz, Alter].

Take for instance, the Müller-Lyer illusion. In a study, most European South Africans thought the lines were of different lengths but bushmen in some South African tribes correctly noted they were the same lengths. Scientists have theorized that people in western societies are used to seeing straight lines and geometric shapes, and people with other cultural experiences aren't exposed to the same geometric configurations, so their brains don't leap to the same conclusions when exposed to illusions that are built upon geometric trickery [source: Schultz].

The "devil's tuning fork" is an illusion where a three-pronged instrument at one end morphs into two prongs at the other.
The "devil's tuning fork" is an illusion where a three-pronged instrument at one end morphs into two prongs at the other.
Wikimedia Commons

However, when test computers designed to mimic the brain's activities were given the same illusion, they are also duped. So the cultural influence on perception of illusions, if it actually exists, is still a big question [source: Schultz].

Most of the optical illusions we are used to seeing, such as the "devil's tuning fork", have been around for a long time. New illusions are largely riffs off the old classics. Even the Op-Art movement of the 1960s and 1970s, which showcased a whole new series of illusions as fine art, used classic notions like the visual influence of adjacent objects, apparent motion, and twists on perspective that many of our old favorite illusions use. People today, however, are still coming up with new takes on optical illusions. Vision researchers hold an annual contest to find the best new illusions. Not only is the contest fun, but it serves to help them study more about how the brain perceives these images.


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