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Optical Illusions: Sights for Sore Eyes

Updated on July 14, 2013

Moving Illusions and More

They've been called trompes de l'oeils - triumphs over the eye. They invite, or rather, force you to take a second, third, or fourth look at them. Some have become famous works of art. Learn about the different types of illusions that can trick your eyes into seeing things beyond the range of normal perception.

This image, for example, isn't moving at all. Don't believe it? Close one eye and focus in on any single colored area. Now what do you see instead?

Illusions were first noticed centuries ago, and have contributed to our entertainment ever since. Keep reading to learn how illusions have changed the world. Afterward, be sure to check out the page that highlights illusions that work because of the physical structure of our eyes and the one featuring perceptual & psychological ones.

Copyright image courtesy G. Sarcone. Click link to view ancient and vintage visual illusions.
Copyright image courtesy G. Sarcone. Click link to view ancient and vintage visual illusions.

First Optical Illusion

Optical illusions have existed since the dawn of time. A distant branch may look like a bird. The moon may look bigger at moonrise and smaller as night plunges on. By definition, an optical illusion, sometimes called a visual illusion, means we see something that differs from reality. This can be achieved using a variety of techniques.

Perhaps the earliest example of a human created example of an optical illusion was an image stamped onto Greek coinage. Believed to date from 550-450 BC, this coin depicts two boars facing each other in battle. Together, they create a ferocious boar's head facing the viewer. (Special thanks to ArchimedesLab.org for this terrific find - it's the only illusion site I found featuring this piece. Click on the picture at right to view their extensive collection there.)

By the 19th century, optical illusions flourished and laid the groundwork for popular inventions and scientific knowledge we enjoy today. Nonetheless, there isn't a universal system for classifying different types of illusions. Should the be described by their impact (double images, camouflage, psychedelic, geometric, and so on) or by what contributes to their effects (visual, perception, or combination?) For the purpose of simplicity, this article will use simple classifications used by Terry Jennings in his introductory book. Jennings uses three basic categories: Sight illusions occur because of the way our eyes work, perception illusions occur because of the way our brains interpret information, and Movement illusions that may involve they way our eyes and brain interact.

Necker Cubes may reveal our ability to focus attention.
Necker Cubes may reveal our ability to focus attention.

How Eye Illusions Work

Sight-based Illusions

One of the earliest and most simple illusions was illustrated by a line drawing of a cube. Louis Necker, a Swiss crystallographer, first published the transparent cube in 1832, asking people which cube face was closest to them. Because there were no clues enabling their depth perception to guide them, viewers saw the lower left square as being nearest, or the top right one. Some people perceived the two squares competing for frontal position.

Today, scientists believe that Necker cubes can reveal information about how we store memories and demonstrate our ability to direct attention effectively. They measure how often subjects see the frontal face change, something that should be tested after head injuries or when patients are experiencing severe stress or major disease according to the American Journal of Nursing.

Hermann Grids

This scintillating grid, based on work published by Ludimar Hermann, in the 1870s, has white dots placed where the lines intersect. Hermann grids don't have dots added, and a viewer would see dark circles appear where the lines crossed. In the scintillating grid, the white dots contribute to a sense that white and black dots are both present and in motion.

This effect is due to the way photoreceptors in the retina work. Some of these cells activate when exposed to light, and others are designed to respond to an absence of it. Black color lacks light-reflecting pigment, so it activates the dark-oriented photoreceptors. When both types of cells activate at the same time, they're forced to compete. A process known as lateral inhibition takes place as one tries to dominate the other, which can make our eyes misinterpret what we see.

Thaumatropes

Moving Pictures Books and Toys

During the Victorian era, thaumatropes were a popular optical illusion toy. Invented in 1824, these simple devices consisted of two cards with two completely different (but related) images. When the cards are spun rapidly, the pictures merge to create another image.

The image shown here illustrates a thaumatrope with an empty birdcage on one side of the card, and a bird on another. When an elastic band is threaded between the cards to form handles for spinning, the little boy sees a bird in a cage.

Thaumatropes demonstrated the idea of persistence of vision. It was believed that the eye's retina retained an afterimage of what it saw briefly, though the theory was debunked in 1912. Nonetheless, thaumatropes led to other persistence of vision toys and laid the groundwork for cartoons and cinematography. Fantascopes, daedalums, and phasmatropes are just a few of the persistence of motion toys that developed after the invention of thaumatropes and contributed to the development of cameras that could capture images in sequential frames to reproduce movement.

Making Movies Without Pictures

These simple illusions are based on the same principles as thaumatropes. Can you think of one you could create?

Persistance of Vision

Phi Phenomenon

When the persistence of vision theory was disproved, science sought to explain how our eyes and brain work together to create an illusion of persistent movement rather than perceiving a sequence of separate images. Known as the "phi phenomenon," the principles that underlie movie-making continues to be called persistence of vision, even though the original meaning of that phrase (that our retinas capture and retain an image after we've stopped seeing it) has been shown to be false.

First described by Max Wertheimer in 1912, the term phi phenomenon actually includes two closely related types of movement perception: phi and beta. Beta perception is what we see when we read moving letters on a light emitting diode (LED) display on our bank's marquee. The letters appear to move across the surface, even though we're simply seeing a series of flashes of lights. The actual position of the lights never move, even though different bulbs light up in sequence to create the illusion of movement. Phi phenomena are similar, and even experts have trouble classifying the difference.

Phi results from images that have no contour, always take on the color of their background, and are presented in faster sequences than is visible with the beta phenomenon. Notice the image here of lilac colored dots against a gray background. Twelve of these images, when flashed quickly, make it look like a gray dot is moving across each of the lilac points. (Click on the image to view it in motion.)

Though poorly understood a century after it was first described, the "apparent movement" created launched the Gestalt movement in psychology. One group of researchers at Purdue University have put together an explanation of the difference (with examples) that can be viewed by clicking the link below.

Cognitive Illusions

Optical Illusions with Color

Our brains were probably intended for tracking movement, as the successful hunter must do if he wants to feed his tribe. This ancient characteristic is deeply ingrained, and makes us vulnerable to a variety of illusions. Cognitive illusions may be based on distortions, ambiguity (illusions that can be one thing or another), paradoxes, or fictions.

Distortions rely on our depth and color perceptions, combined with the way our brains work. In this image, for example, the squares marked A and B are exactly the same shade of gray.

Don't believe it? Our brains tell us that squares that are the same color should look the same. In fact, because it's shadowed, square B should appear darker than square A. By introducing a contrasting color and shadow, our mind is fooled.

Image courtesy of Edward H. Adelson
Image courtesy of Edward H. Adelson

Mind Teaser Illusions

Make Optical Illusions with Photoshop

By using a program like Adobe Photoshop or Corel Draw, it's possible to match the color of either square and connect them using the matched color.

What do you see now?

Essentially, our brains see what they anticipate. The same process is at work in distortion illusions.

Schizophrenics may be immune to this effect and see both shades accurately. To learn how illusions may detect undiagnosed schizophrenia, visit thy psychological illusions page I mentioned earlier.

Though it may not look like it, the lines separating rows of tiles are straight.
Though it may not look like it, the lines separating rows of tiles are straight.

Optical Illusions Drawings

Optical Illusion Art Relies on Colors and Shapes

Mark Changizi, a researcher at Rensselaer Polytechnical Institute, believes a perceptual lag is responsible for the way our brains misinterpret straight lines as bent in this illusion. Science has long recognized that our brains don't interpret what the eye sees until about a tenth of a second after an image hits our retina. Changizi proposes that our brains compensate for the lag. "Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality."

Changizi believes that our brains anticipate what the image will be when we move toward it. The effect can be seen with illusions like this one, known as the "cafe wall" because it was first noticed on... yep, the wall of a cafe in Bristol, England. The wall wasn't really poorly built - it just looked that way.

Many illusions are successful because of the way our visual system is structured. To see many fascinating examples and learn more, visit Sight Based Illusions.

Salvadore Dali used optical illusions in many paintings.
Salvadore Dali used optical illusions in many paintings.

Optical Illusions for the Brain

So far, we've looked at illusions that are the result of how our visual receptors can be fooled, and the way our brains and eyes interact to see something clearly and yet not see or understand it. The third basic category of illusions involves those clever images occur when our eyes accurately see and transmit data to our brains, but our brain believes it's seeing something else.

M.C. Escher, Salvador Dali, Bev Doolittle, and other well-known artists create works that tantalize our brains by showing us one thing while our mind sees another. Here, fans may see a man's profile or a couple pausing in the lane. (Look closely to see more illusions.)

This summary about the types of optical illusions describes how our eyes and brain work by themselves and together to process visions that are something other than what they appear to be. For more examples of illusions like these and how they're being used by marketers to influence our thinking, check out this page.

More examples

Click thumbnail to view full-size
The Necker cube effect is seen in this illustration, offered by Jie Qi. Source: Jieq, Flickr.Carl Glover provides this outstanding example of a Necker cube illusion. Source: *KingOfTheAnts*, Flickr.The colors of this scintillating Hermann grid photo make it extremely interesting. Source: Morpholux, Flickr.Not all Hermann grids produce the same scintillation effect, but the black dots still appear in a regular Hermann grid illusion. Source: Richard Cocks, Flickr.Persistence of vision has been used to produce toys and consumer objects, one of which is seen in Micah Elizabeth Scott's photo. Source: Scanline, Flickr.Thaumatropes could be easily enclosed in mail to send a little love. Source: Kate Hartman, Flickr.Marcin Wichary's photo of the bird cage thaumatrope reveals one way they were used. Source: Marcin Wichary, Flickr.
The Necker cube effect is seen in this illustration, offered by Jie Qi. Source: Jieq, Flickr.
The Necker cube effect is seen in this illustration, offered by Jie Qi. Source: Jieq, Flickr.
Carl Glover provides this outstanding example of a Necker cube illusion. Source: *KingOfTheAnts*, Flickr.
Carl Glover provides this outstanding example of a Necker cube illusion. Source: *KingOfTheAnts*, Flickr.
The colors of this scintillating Hermann grid photo make it extremely interesting. Source: Morpholux, Flickr.
The colors of this scintillating Hermann grid photo make it extremely interesting. Source: Morpholux, Flickr.
Not all Hermann grids produce the same scintillation effect, but the black dots still appear in a regular Hermann grid illusion. Source: Richard Cocks, Flickr.
Not all Hermann grids produce the same scintillation effect, but the black dots still appear in a regular Hermann grid illusion. Source: Richard Cocks, Flickr.
Persistence of vision has been used to produce toys and consumer objects, one of which is seen in Micah Elizabeth Scott's photo. Source: Scanline, Flickr.
Persistence of vision has been used to produce toys and consumer objects, one of which is seen in Micah Elizabeth Scott's photo. Source: Scanline, Flickr.
Thaumatropes could be easily enclosed in mail to send a little love. Source: Kate Hartman, Flickr.
Thaumatropes could be easily enclosed in mail to send a little love. Source: Kate Hartman, Flickr.
Marcin Wichary's photo of the bird cage thaumatrope reveals one way they were used. Source: Marcin Wichary, Flickr.
Marcin Wichary's photo of the bird cage thaumatrope reveals one way they were used. Source: Marcin Wichary, Flickr.

Thanks for Seeing & Sharing

A special thanks to these folks who have helped this page succeed:

Michey

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    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      Thanks for checking it out, mel-kav! I have a few other lenses with many other types of illusions, too. If you get time, I hope you'll check them out.

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @mel-kav: Thanks for stopping by! Hope you'll check out my other lens on illusions that are purely created because of how our eyes work.

    • mel-kav profile image

      mel-kav 5 years ago

      Very Interesting. I love optical illusions!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @Ahdilarum: Thank you!

    • Ahdilarum profile image

      Ahdilarum 5 years ago

      Well, now you may be happy with the way of traffic to this lens. Great one.

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @Countryluthier: Hmm... Would a brain turbocharger resemble a tin foil hat? I have one of those I could lend you so we could try!

    • Countryluthier profile image

      E L Seaton 5 years ago from Virginia

      Simply fascinating, can I turbo charge my gray cells and beat these? Great lens!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @anonymous: Thank you, Dave!

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      anonymous 5 years ago

      Cool lens...lotta fun! The intro photo is remarkable!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @uAttendUK LM: Thank you!

    • uAttendUK LM profile image

      uAttendUK LM 5 years ago

      Reading your lens was really a great fun and it's very informative. Nice effort

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    • greenmind profile image

      GreenMind 5 years ago from USA

      Read your SquidU Forum exchange -- I like this lens even though it will never sell a single Wii console or fitness machine. Maybe because of that.

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @BenJacklin LM: Grin... Thanks, Ben. And that sounds like a great plan!

    • BenJacklin LM profile image

      BenJacklin LM 5 years ago

      Amazing and informative, and now I need a lay down...

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @favored: Aw, thanks so much! I have been debating whether to write the last two in the series, since they haven't drawn much attention. You've given me some motivation.

    • favored profile image

      Fay Favored 5 years ago from USA

      Absolutely loved this. You still have my eyes going. Your intro pic was perfect. I used to use these with my students and they always amazed us all. So glad you chose this topic. Adding it to my favored1 lens if it's ok with you.