Our Favorite Viral Optical Illusions Explained What’s not to love about optical illusions? The magical, mysterious, mind-bending visuals never cease to evoke a series of ‘“woahs” and “what the’s--.” Words usually fail to describe them. Yet, one common word that is often used is trippy, hearkening to the psychedelic days of the ‘60s and ‘70s. But, believe it or not, the first optical illusions emerged far earlier, in the 5th century B.C. They even had Aristotle bumfuzzled! From Medieval Latin, opticus means “of sight or seeing” and ludere, “to play”—thus, optical illusions are visuals that essentially “play with one’s sight.” Because these images are invested with such mysterious power, we think optical illusions are some of the coolest didactic devices known to man. They’re brilliant to look at, and they teach us to question everything. So, to honor these powerful images, we want to share with you their proper names—because, honestly, trippy only holds up in the basement. Get your brains and eyeballs ready. They will be boggled. What are lightness or luminance illusions? Another group of illusions is known as lightness or luminance illusions, which explore how the human eye perceives lightness (the degree of brightness or shadow) reflected off a surface. This includes the Checker-Shadow Illusion. Which square is lighter in the image, A or B? This illusion was created by neuroscientist Ted Adelson in 1995. The illusion is named for the apparent shadow, created by the green column, that falls across the checkerboard pattern. The square marked A looks significantly darker than the square marked B, but both are the same shade of gray. The tiles have the same luminance (scientifically, the same number of photons reflecting off the them). There are a ton of photo illusions that went viral online and demonstrate this same luminance phenomenon. Including that black and blue dress from 2015 ... we know you remember that one. We'll get to that soon. Vans Shoes—Pink and White or Gray and Mint (or Teal)? This photo illusion exemplifies color constancy. Eyes have to compensate for a lot of visual noise. Here, the lighting gives a turquoise cast to the whole picture, which is why some people perceive the shoes as gray and mint/teal. But, the pink-and-white camp are able to compensate for the turquoise filter and figure out from other clues (like most shoelaces are white) that the Vans are actually pink and white. The same explanations apply to this next viral illusion ... Are these strawberries red? There's not a smidgeon of red in the picture. The pixels are actually blue and gray—the whole image is awash in that turquoise color filter. But, because strawberries in nature are red, the human eye color-corrects the image and thinks the berries are actually red. This illusion was created by Akiyoshi Kitaoka, a psychology professor and optical illusion artist, in 2017. Basically, beware Instagram filters next time you see another photo of a luminance illusion! #The Dress—Black and Blue or White and Gold? That leads us to this photo that went viral in 2015 after a Tumblr blogger posted the pic and sent everyone and Tailor Swift into a tailspin. The actual dress is black and blue, but some people perceive it as white and gold in a similar way people think A and B in the Checker-Shadow Illusion look like different colors, even though they aren't. It's a matter of perception, luminance (how light is reflecting off the object), and context. The surrounding color-context influences the way the focal colors are perceived. The lighting in the pic makes it seem as if the colors of the dress are different than what they are, at least to some people. Apparently, this illusion is so striking because, unlike the Checker-Shadow, not everyone perceives the color difference—which is why it was the subject of a raging debate. What are autostereograms? Is this image 2-D or 3-D? These optical illusions are especially trippy, with highly patterned images in a rainbow of colors. Called autostereograms, the visuals are made up of a series of repeated 2-dimensional images that hide a 3-D form. Remember those posters and books from the 90s? Even if you didn't see the image, you told all your friends you did, right? Of course! But, this is how it could actually happen: soften your gaze on the picture (maybe do a little cross-eye dance) or focus your eye on a point just beyond the image. A 3-D shape should appear to “pull up” from the background. Voila. Here's some of the best-selling Magic Eye memorabilia for you to go cross-eyed over: Disney's Magic Eye ... because Disney. Harry Potter Magic Eye Book ... We bet you didn't know this existed! Magic Eye: A New Bag Of Tricks ... because Magic Eye never goes out of style. This kind of optical illusion tests your stereopsis, or ability to perceive depth, which is important when driving and parking, pouring liquids in vessels, climbing stairs, and reaching for objects. Here’s an example where the 3-D form hidden in the pattern looks like the outline of a shark. Can you see it? What is the Café Wall Illusion? Are the lines parallel or angled? The Café Wall Illusion was first discovered in the late 1890s, but rediscovered by British psychologist Richard Gregory in the 1970s. The illusion is so-named because Gregory noticed a peculiar pattern in the brick wall of a café in Bristol, England. The pattern, replicated above, consisted of staggered rows of checkerboard blocks. When viewed from a distance, the mortar (here the gray horizontal lines) appeared to angle upwards or downwards, even though the lines were (and are) completely parallel. Philosophers debate on what this illusion demonstrates about human experience, how humans can hold on to two contradictory ideas at once—knowing the lines must be parallel, but experiencing them as something different. Here's a more recent version of the illusion that went viral online in 2017 after it was a finalist for "Best Illusion of the Year." This revamped classic is even more brain-breaking with the added diamonds and squiggles. What is the train track illusion? Which track is longer? The internet went crazy over this train track illusion in 2016. It looks like one of the pieces must be longer than the other, but both are the same size! This is an example of the Jastrow Illusion, discovered in 1889 by psychologist Joseph Jastrow. Basically, the long-and-short of it (ha, we kill ourselves) has to do with how identical curved objects are placed in relation to each other. In the illusion, the long side of one track is next to the short side of the other, and that's why the tracks look different in size until—brain melt—the two become one ... just like the Spice Girls sang, that's right. What are illusory contours? How many triangles are there? This first optical illusion illustrates an example of illusory contours. Smaller, seemingly incomplete shapes are configured in such a way that the brain perceives contours, outlines, or edges, of larger shapes. So, for example, the set-up of Pacmen and half-triangles in the image above magically form two larger triangles in the brain. But, the large triangles aren’t actually there! The brain is wired to fill in the gaps according to surrounding visual clues. Eyes take in a lot of visual information at once under different conditions (e.g., lighting), but the brain has the ability to complete a visual, even when only given partial information. A car halfway in a shadow is still a car in the mind, not half a hunk of metal. In the early 1900s, a man named Friedrich Schumann officially discovered illusory contours, but the optical illusions appear in art from the Middle Ages. Here are some other examples: What are impossible objects? There are many examples of what are called impossible object illusions, in which the viewer is presented with a picture of an object that couldn’t possibly exist in real life because the lines don’t connect: what’s front becomes back, what’s up is suddenly down, and pathways lead nowhere or repeat themselves in a fantastical loop. It’s also called an “un-decidable figure” because the viewer can’t make heads or tails of it. This includes the impossible trident ... How many prongs are there in the image? One of many examples is the Impossible Trident, which also goes by the names Impossible Fork and Devil’s Pitch Fork. The object resembles a 3-pronged instrument (fork) or weapon (trident), but nothing lines up as it would in reality. If you focus on the left side of the image, the object looks 3-pronged, but cast your gaze to the right, then suddenly the instrument has only 2. Because of its wicked visual-trickery, the Impossible Trident is sometimes named for the Devil (because surely, that’s what the pitchfork from hell would look like, right? Always keeping sinners guessing how many prods they’d get). The illusion also goes by the name Blivet, which means “something annoying, ridiculous, or useless.” It also includes ... the Penrose Stairs Where do the stairs lead? Called Penrose Stairs, this impossible object illusion was named after its creators, mathematicians Lionel Sharples Penrose and his son Sir Roger Penrose, in 1958. The stairs appear to be ascending in one direction and descending in the other, and yet one loop around brings you back to your starting square. And, the Belvedere Is this a building? The artist M.C. Escher was known for his interpretations of impossible objects on a grander architectural scale. This illusion is called Belvedere. In Italian, belvedere means “beautiful view,” and it is an architectural term for an elevated open-air structure. Escher’s drawing depicts a belvedere with impossibly-placed pillars, ladders, and staircases—and yet the assemblage as a whole also appears to be a strangely functioning building. What are autokinetic illusions? Is the image moving? Formed by the fusion of Greek auto- (“self”) and kīnēsis (“movement”), an autokinetic illusion is one where the stationary image appears to move on its own. The phenomenon was first recorded in the late 1700s by a German naturalist named Friedrich von Humboldt. He noticed an illusory motion of the stars when looking up at the night sky (he also called the phenomenon "star swinging"). Another kind of illusory motion written about in German is "dot swinging," a phenomenon in which black dots appear to move against a white backdrop. Then, there's the Rotating Snakes Illusion ... beware. The autokinetic illusion above is known as the Rotating Snakes Illusion, created by Kitaoka in 2003. A series of identically patterned concentric circles (with token red forked tongues) seem to slowly rotate. The trick to seeing the movement is to allow the eye to flick and wander across the image—once you fixate on any part of the visual, the snakes stop slithering. Aw, and Rollers! Here’s another one of Kitaoka’s autokinetic illusions, called Rollers (presumably because the configuration of dots roll against each other, and they may remind some viewers of hair rollers). And, finally the Scintillating Grid What colors are the dots? A similar illusion involving hallucinatory motion (and, here, color change) is called the Scintillating Grid, and was created in 1995. In this example, as you move your eyes across the grid, or lattice of vertical and horizontal lines, the dots at the center of each intersection appear to flash or scintillate. All dots are white, but as the eye scans, the peripheral dots (the ones the eye isn’t focused on) appear to turn black, and the perceived vacillation between black and white dots creates a sparkly scintillating effect. This version of the Scintillating Grid went viral in 2016, after it was posted on Reddit and Twitter: Instead of experiencing a sparkly scintillating effect, though, this illusion plays with your peripheral vision. No matter how hard you try, you won't be able to see all 12 dots at once. Perception scientists call this the extinction effect. The white border around the black dots makes the dots appear lighter (or gray) when viewed peripherally. Against the gray channels of the grid, the dots disappear. Both variations of the Scintillating Grid are based on a class illusion called the Hermann Grid, created by German physiologist Ludimar Hermann in the 1870s. What are ambiguous figures? What image do you see? These last examples are probably some of the most well-known optical illusions. They go by different names, including bistable illusions and ambiguous figures. The word bistable is an electronics term that describes the condition of having “two stable states” in an electrical circuit. In optical illusions, one image evokes two mutually exclusive stable images within it. So above, the image can be perceived as a black vase on a white ground or two white faces on a black ground. The same configuration of lines and shapes can evoke two (or more) different entities, but not simultaneously. In our brains, only one entity can achieve perceptual dominance at a time. These forms are also described as ambiguous figures, where the figure is in an “uncertain” state in our brains, neither completely one entity, nor the other, nor both, but somehow, contradictorily, all three. Yet another way to describe this illusion is as a Gestalt shift. In German, Gestalt means “shape,” so figures displaying this phenomenon are, essentially, “shape-shifters.” Or, rather, humans make shifts in their perceptions to see the different gestalts. You've already seen another example of this illusion, the Impossible Trident with three and/or two prongs. Coming up are two more classic examples ... The Young-Old-Woman (or, My Wife and My Mother-In-Law) The Young-Old-Woman Illusion (also called My Wife and My Mother In-Law) is an illusion that appeared on German postcards and in an A & P Condensed Milk ad in the 19th century. Its first reference in a scientific context was in 1930. The version here was created for Puck magazine in 1915 by British cartoonist W.E. Hill. One illusion title is more descriptive (i.e., laying out the fact that the face is at once female, young, and old); the other title is cheekier, in praise of a beautiful young wife and poking fun at the wife’s haggard mother (in-laws have always been hard to deal with, apparently!). All is Vanity All is Vanity is another popular ambiguous figure or bistable illusion, created in 1892 by an American illustrator named Charles Allan Gilbert. From one perspective, the image depicts a lady getting ready at her dressing table. From another perspective, the mirror, the woman’s figure and her reflection, form a giant skull. The illusion is playful both visually and linguistically; the viewer is led to understand the woman’s attentions to her physical appearance at her vanity table contributes to her emotional vanity and excessive pride, which will lead to her downfall. These two classic examples show how illusions can be used as messages to teach viewers important life-lessons. They are mysterious reminders that everyone must age and die, and it’s a waste of precious time to place too much emphasis on the superficial things in life. Those things are mere illusions, anyway!