Figuring Out These Puzzling Optical Illusions Is Almost Impossible

Stare at this enigmatic image for a while and you’ll get a surprise if you focus on it in just the right way. Not everyone can see the likeness of a multicolored shark hidden among the seemingly random pattern of dots. It’s all a question of throwing your focus off to reveal the fish, which emerges in three-dimensional glory. Magic Eye books caused a sensation in the 1990s, dividing people into those who could see the concealed objects and those who couldn’t.

Although this image shows you the shape of the shark, it is a pale imitation of what it’s possible to see in the full picture. The magic works because of the fact that humans have two eyes. Each sees a slightly different view, but the brain knits the image together. A Magic Eye picture exploits this difference with its arrangement of dots. If you focus through the picture you’ll see the concealed object within it. 

It was a British psychologist called Richard Gregory who first named the Café Wall phenomenon. Why did he settle on the name? Well, it was because he had literally spotted the illusion in the brickwork of a café in the English city of Bristol. Look at the lines running across the picture. They’re not parallel are they? Oh yes they are — prove it to yourself with a ruler. 

It’s the pattern of the bricks that makes the lines appear to be off-kilter. And here’s the late Professor Gregory outside the café where he spotted the illusion. Precisely how it works is not fully understood. The Illusions Index website says that “it appears to involve interactions between the neurons in the visual cortex which code for orientation.” Hope that clears things up.

Just try to count this elephant’s legs: it’s guaranteed to drive you nuts. Of course, we all know full well the number of legs the beast should have. Four, of course. But as you attempt to count them, they seem to dodge across the drawing. But here’s a top tip. Cover over the elephant’s feet, and the image will settle down to show the expected number of limbs. 

Now here’s a picture of a real pachyderm. There’s no problem counting the legs here, is there? But the first sketch plays a trick which bamboozles your brain. It’s quite simple really — the artist has drawn the feet in the spaces between the legs, unattached to the limbs. Roger Shepard is the fiendish trickster behind this illusion. We’ve got to give it to him — it’s an absolute doozy. 

Stare hard at this apparently random set of black-and-white blobs for about a minute. Then look at a blank surface. You should see a human face that’s very familiar: Jesus Christ himself. The amorphous shapes have transformed into a coherent image. Your eyes and brain have made sense of the picture. 

So how did that happen? The Britannica website has an explanation: you’ve experienced something called an afterimage. The encyclopedia explains that this illusion results when “retinal impressions persist after the removal of a stimulus, believed to be caused by the continued activation of the visual system.” Stare at the image above for a time and it, too, should give you an afterimage — with the colors flipped.

There’s something most definitely not right about this strange cube: it appears to defy the normal logic of geometry. The famous Dutch artist M.C. Escher created this illusion, notably using it in a 1958 work titled Belvedere. It’s a powerful illusion — you could spend a lot of time trying to unravel exactly what’s happening. We have!

Even once we know that this two dimensional series of lines could not possibly exist in three dimensions, it still baffles us. So what’s going on? In simple terms, our brains are trying to make sense of this impossible cube. But it defeats our normal unconscious cognitive abilities and leaves us bemused. Our conscious mind just can’t work it out. 

This extraordinary image, named Rotating Snakes, creates a highly captivating illusion. The circles appear to rotate, rather like a bunch of serpents writhing around. You could almost swear that you were looking at a video clip. But trust us, you’re not. The movement you see is entirely in your own mind. Your eyes are playing a trick on your brain. 

What makes the illusion of movement feel so real, even when we know it’s not really there? Speaking to radio station NPR, vision researcher Professor Ben Backus gave his explanation. It’s all to do with the way the retina gathers information to feed to your brain and how your neurons then react. It’s rather complex, but suffice to say that the way your neurons respond to the colors in the image causes the illusion.

Look at these lines. The upper one is divided in two by the slanted lines. The left-hand sector is clearly longer than the right-hand side. Now take the lower two lines. Again the left hand section is longer than the right, yes? No! All four lines are exactly the same length. Doubtful? Check it with a ruler! You’ll be surprised...

A German academic called Franz Carl Müller-Lyer created this illusion in 1889. The apparent disparity in the line lengths is created by the positioning of the arrowheads. You’ll notice that when they’re pointing outwards from the line, that appears to shorten it. When they’re pointing into the line, that seemingly lengthens it. Ultimately the slanted lines are fooling your perceptions in a surprisingly convincing way. 

Take a glance at this work and you’ll likely see one self-evident image. But examine it for longer and you should begin to spot that there are actually two entirely different ways to see this picture. Indeed there are two different women portrayed here and they could hardly be more contrasting. There’s an older lady of rather unflattering appearance, but then there’s a younger, attractive woman.

If you’re struggling to see both likenesses, the older woman’s nose is the younger woman’s partial profile. And the younger woman’s choker is the older woman’s mouth. One interesting piece of research at Flinders University in Australia found that older individuals saw the “mother-in-law” first. But younger people initially catch sight of the “wife.” The duck/rabbit image above is a similar type of illusion.

We have a German psychologist called Hermann Ebbinghaus to thank for this particular illusion. As you can see, we have two orange dots in the illustration, each surrounded by blue dots. On the left we have six large blue dots and on the right eight smaller ones. And the orange dot on the left is clearly larger than the one on the right. 

But despite appearances, those two orange dots are actually precisely the same size. As are the two central blue dots in this image above. This is another case where the evidence fed to our brains by our eyes is misinterpreted. And it’s a powerful misconception. Even when we know the central discs are actually the same size, that’s not the way it looks!

Now here’s the Hermann Grid playing tricks on your perception. Notice the grey dots you can see as they appear intermittently in the intersections of the grid? They’re not actually there at all. Stare closely at one intersection and the dot disappears. But you can still spot them in other intersections in your peripheral vision. At least, that’s what happens to us. 

The Illusions Index website tells us that, “The Hermann grid is a classic example of what vision scientists call a simultaneous lightness contrast illusion.” But some philosophers, apparently, have a somewhat different view. They have questioned whether the grey dots are an illusion or a hallucination. If that helps you to understand the Hermann grid, well done you. And just to confuse matters further, the picture above shows white dots that are there flipping to black in your peripheral vision. But they’re definitely still white.

Dario Varin, an Italian psychologist, introduced the world to the neon color-spreading illusion in his 1971 paper Chromatic Contrast and Diffusion Phenomena — no doubt it’s a fascinating read. What this illustration shows is a neon color circle. We see a circular disc thanks to the blue coloring of the otherwise blank lines. The disc is not actually there. 

The Illusions Index website frankly admits, “The physiological mechanisms underlying neon color-spreading are not entirely understood.” The best the experts seem able to come up with is that this is an example of “filling in.” That’s when the brain takes messages from the eye and basically creates a reality that makes some kind of sense to the individual. Probably.

Here’s an illusion to really tangle and confuse your neurons. The two pieces of railway track in the image above clearly appear to be different lengths: the upper one looks shorter than the lower. If only that were true. But the fact is that the two pieces are entirely identical in size. Yes, we know that seems incredible.

However, look at this image and you’ll see that our assertion is true. The two pieces of toy railway match exactly. It was American psychologist Joseph Jastrow who first noted this illusion more than a century ago. What’s happening is that the top corner of the bottom part is aligned with the bottom corner of the upper piece. And because the track curves the bottom of each piece is shorter than the top. So despite appearances, the bottom piece isn’t bigger, it’s just shifted to the right.

It’s no surprise that the Penrose Triangle is also known as the Impossible Triangle. Just take a look at it — defying logic, it appears to make no sense whatsoever. Swedish artist Oscar Reutersvärd is said to have created this puzzler in 1934 when he was just 18 and bored in a Latin lesson. Well, Reutersvärd may have discovered the triangle, but it was a father and son duo, the Penroses, who brought the triangle to wider popular attention in the 1950s.

The point about this triangle, or tribar, of course is that it could not actually exist in three-dimensional reality. As the Illusions Index website points out, “the bottom bar of the tribar is represented as being spatially located to both the front of, and, at the same time, the back of the topmost point of the tribar.” And that is impossible. Yet we can still see it when it’s rendered in two dimensions.

So, the question is, what do you see when you look at this image? As it’s titled Rubin’s Vase, you very probably see a vase on first glance. But look again, concentrating on the black part of the picture, and you’ll see something quite different: two human faces looking at each other. In other words, the single illustration contains two quite different images. This illusion was created in 1915 by psychologist Edgar Rubin, a Danish psychologist.

Rubin’s Vase is an example of what’s known as an ambiguous figure. There is no universal agreement about how exactly this illusion works. Although it’s clear that the eye is seeing a single image, the process of the brain flipping from one view of the illustration to another — vase or faces — is not fully understood. So perhaps the best advice is to relax and enjoy the ambiguity.

You’ve certainly heard of both silver-screen star Marilyn Monroe and 20th-century genius Albert Einstein. But a composite figure of the two is likely much less familiar. But here it is, a single image of each combined to create a somewhat nightmarish character, Marilyn Einstein. It comes as something of a surprise to learn that researchers created this image as an aid to the diagnosis of sight problems.  

The researchers who created the image deliberately used a blurred photo of Marilyn combined with a sharper image of Einstein. So those with weaker eyesight will actually see the less defined picture of Monroe, since the poorer vision is less good at seeing the detail of Einstein’s image. Those with good eyesight can expect to see the actress from a distance, but Einstein in a closer view.

If you’re prone to seasickness, you might want to exercise a little caution while you look at this one. That’s because the illusion of movement created by the blue dots on their green background can make anyone feel a little queasy. But if you can relax into it, the movement can actually be almost soothing.

This phenomenon is known as illusory motion — and that term describes the experience well. The image is in fact static, but our brain perceives movement. It happens when the retina sends information to the visual cortex in the brain. Neurons are activated first at high speed, then more slowly, in a process called adaptation. But the rate at which the neurons fire change speed varies with different colors, causing a perception of movement.

Now we’re asking you to give the red dot in the middle of the blue circle a hard stare. After a time, the blue surrounds will start to fade and eventually disappear altogether. This is the Troxler Effect and it’s named after Ignaz Troxler, a Swiss doctor and philosopher who discovered the phenomenon in 1804.

With this image, if you stare hard enough at it for long enough, the color differences will merge into one single hue. The experience is explained by what happens when you fix your eyes on a particular spot. The Illusions Index website explains that “any constant light stimulus will cause an individual neuron to become desensitized to that stimulus, and hence reduce the strength of its signal to the brain.” And so parts of an image will seem to merge or disappear.

Take a look at this illustration of a grayscale checkerboard with a green cylinder set on its edge and casting a shadow across it. As you can see, one of the squares is marked “A” and a second “B.” And it’s quite plain the former is quite a lot darker than the latter. Or so it seems. In fact, these two squares are exactly the same shade of grey. 

Edward H. Adelson created this puzzling illusion in 1995 and its secret is something called light constancy. That is the way that we perceive light-toned objects in dim light as opposed to the way we see dark objects in bright light. The property of light constancy means that your brain is tricked into believing “A” is darker than “B.”

As you can see, this illustration of a railroad track has two yellow lines drawn across it. The upper line is obviously longer than the lower one. But yet again, our brains are playing a trick on us. As you might have guessed, the truth is that these two yellow lines are exactly the same length. 

Italian psychologist Mario Ponzo first revealed this illusion in a book published in 1910. This picture illustrates a slightly different method of creating the illusion. But the effect is the same, indeed somewhat heightened. Despite our perception, the two pink lines are the same length. This is a geometrical-optical illusion and it’s caused by the way the different lines in the image distort our perception.

This illusion takes us onto city streets to view images created by clever artists. By employing just the right angles, it’s possible to create amazing illusions such as these giant teacups tumbling from a building. An artist with a good understanding of perspective and human perception can create astonishing images.

An image like this is often called a trompe-l’oeil, a French term simply meaning “deceives the eye.” The phenomenon works because of the way that depth of field is created with skillful use of perspective lines. As with so many visual illusions, the eye sees the image and the brain then interprets the information to make sense of it.