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Optical Illusions: Sight-based illusions

Updated on September 11, 2014

Illusions that Work Because of Eye Structure

Optical illusions can work in a variety of ways, and have many classifications. To simplify, illusions can work because of how eyes and vision work, how brains perceive information, and the way our eyes and brain interact. In Optical Illusions: Sights for Sore Eyes I described each type with some simple examples. But the real fun lies in seeing more (of course I intended the pun!) Here, you'll find many examples of visual illusions that rely upon the physiology of our eyeballs: photoreceptors, lens curvature, binocular vision, and blind spots. (NOTE: Some of these images can cause epileptic seizures in people prone to them.) When you're finished checking them out, head over to the psychological illusions page to check out more illusions and discover how they continue to change the world we inhabit.

Moiré refers to an effect we see when lines are almost, but not quite, superimposed. If you had two sections of window screen, you would notice a moiré effect if you slid one over the other. This cone uses a moiré effect to create a three-dimensional effect that appears to be spinning as your eye hits different points near the center of the image.

Even though our eyes are constructed differently than a camera lens, cameras reproduce these effects.

Created by Paul Nasca
Created by Paul Nasca

Central & Peripheral Vision

When our eyes are moving, they take in information from the periphery as well as our area of focus. This can produce interesting illusions like this one, created by Paul Nasca. If you stop moving your eyes, you won't perceive any movement in the dots. However, if you look from one area of the image to another, you'll see a wavy movement.

In the next illusion, you should see a manwhen you shake your head from side to side. This is another example of how our central and peripheral vision can create a perception of something that's not there.

Can you see it?

Move further away and try again if you have trouble. You can also try shaking your head faster, but it may give you a headache!

Did you know that our eyes have a blind spot that constantly creates illusion? It's true! Test it like this: Stand up and hold your arm straight out from your shoulder. Keep it parallel to the floor while you keep your eyes facing forward. Raise your index finger toward the sky, and slowly move your entire arm forward while continuing to look straight ahead. When your finger reaches a certain angle from the eye, it will disappear. You will see the background, but not the finger. Because our eyes lack photo receptors where our optic nerve enters the retina, they can't perceive anything there. Our brains clone the background to compensate for missing information, so instead of seeing what is really there, we see our brain's best guess. The scientific term for the blind spot is punctum caecum.

Rods, Cones, and Tiring

No, this isn't a road test.

The rods in our eyes perceive black, white, and gray tones. They also define shapes. What they don't do is recognize color. That job is left to the cones, which interpret light into three colors: red, green, and blue. Together, these photoreceptors help us distinguish millions of colors and shapes.

If you focus on a moving image for a minute or two, then look at a stationary object, the object might appear to have movement. This may be because the cones of the eyes were overstimulated and grew tired. This effect is called a "motion aftereffect." Without getting scientific about it, there are two other types of aftereffects: Positive and negative. Positive refers to afterimages that retain the color that was seen, and negative afterimages are those that produce an opposite color.

Look at the focal point on each of these images (stare at the center if you don't see a marked point) for 15-20 seconds, then look at a white space on a wall or piece of paper.

The photo below provides a white space for you. It appears to be in the public domain (if anyone knows that it isn't, I'll remove it if you let me know!) It's a negative image of actress Deepika Padukone but looks like another famous person in the afterimage. Using the same method of staring at the focal point for 15-20 seconds before turning to the white space, who do you see?

Can't Get Enough? - Check these out...

This image created by Wikipedia user Famousdog.
This image created by Wikipedia user Famousdog.

Other Tricks Our Eyes Play on Us

Our eyes interpret vertical distances differently from horizontal ones. This contributes to misperceptions about size and/or distance.

Joseph Delboeuf was a psychologist whose work encompassed focused on designs that were opticogeometric (a fancy word for the way our eyes interpret geometric shapes). His work has been reexamined recently as a means for losing weight. Koert Van Ittersum, a researcher at Georgia Tech, says the Delboeuf effect can affect our weight.

During one experiment, he and fellow researcher Brian Wansink demonstrated that when using smaller bowls, study participants believed they'd eaten more than they had, and when using larger bowls, they didn't feel quite so full and ate as much as 13% more than they intended to!

An article on NPR.org reported the study and described that pouring the same amount of liquid in differently sized drinking glasses can produce the same effect. (For the link and to read other tricks for using optical methods for weight control, visit the link below.)

In this image of concentric circles, you can see the Deboeuf effect for yourself. Both of the solid black circles are the same size, but the one on the left initially appears smaller.

Vertical distances are interpreted as longer than horizontal ones by our eyes, too. When you see the image on the left (below), it appears to have a slightly greater vertical axis, but by examining their measurements at the right, it's easy to see they're the same length.

The Mueller-Lyer effect shows another perspective error. Both lines are the same length. Studies have shown that different cultures are more or less prone to seeing them as different - urbanites are more susceptible than people who live in rural environments, for example. What do you see?

A similar effect can be seen with these arches. Again, they are the same size, but our perspectives make them appear otherwise.

Not sure? Use a piece of paper to line up the bottom left corner of one and compare it to the other. Do the same on the right side.

Luminance

Dynamic Illusion
Dynamic Illusion

PosterPie.com shares an illusion that demonstrates the principle of dynamic luminance. Stare at the center of the photo above. Without blinking, slowly move closer to it. You'll notice the golden yellow area seems to grow, while the brown ridges fade. When you move away from the photo, the opposite happens.

Luminance means "brightness." In the photo above, the yellow area appeared brighter as we moved closer, and darker areas crowded into that area as we moved away, even though the actual brightness of the colors did not change - unless you changed your monitor settings while you were trying the illusion! The way our eyes interpret and adjust to light can create interesting effects like the one you've just seen.

Equiluminance refers to something in which all the colors are the same exact hue, but depending on how those images strike our eyes, we may seem them vary from each other. For a wonderful interactive example, visit a>example that shows how moving red dots can look like a revolving sphere. While there, use the arrow buttons to increase speed and change the background colors to see that they are all one color of red, but can look as if some are dark and some light at the same time.

One last example of luminance and how it can affect our interpretation of colors can be seen in the Munker illusion above. Although it appears to contain at least two different shades of red spirals, there is actually only one. The image below, as well as other examples of Munker illusions, can be found from Akiyoshi Kitaoka. To prove to yourself that they are indeed, the same color, another visit to one of Michael Beck's pages will let you test the colors.

Copyrighted Image, Patrick Trotter
Copyrighted Image, Patrick Trotter

Perspectives, in Greater Depth

(Yeah, ok, puns are fun, dammit!)

Perspectives could be argued to be effective because of the way our eyes work or the way our brain interprets information. Although they probably better fit into the second category, they're so fun that I'll use some of them here anyway.

Patrick Trotter's "Cat Dance," featured here, shows a black feline that is either facing you or walking away. For a closer look or to purchase Trotter's print as a greeting card or a full-sized piece for your wall, click on the image. It will take you to his gallery page.

Graphic editing programs allow perspectives to be distorted to create all kinds of mind-blowing illusions. Perhaps you've seen this one before:

Artist Julian Beever has gained an international reputation for his use of perspective in his artwork. Using chalk as his preferred medium, he creates illusions that have been shared around the world. You may have seen examples of his work in your e-mail inbox. This simple sidewalk and wall now appears to be a convincing superhero scene featuring Beever:

Take a look at how the eyes play tricks on the viewer by comparing these two photos of another of Beever's famous creations:

Great Conversation Starters for Your Living Room

Enjoy art by the two most renowned chalk illusionists, or learn to manipulate perspective in your own artwork. I keep a coffee-table book of illusion art that has stimulated dozens of conversations.

What Your Eyes Don't See

That last kind of illusion based on how our eyes work involves what is invisible to our eyes.

We can perceive shapes that don't exist and images that aren't actually defined because of the way our eyes process light. This photo, for instance, doesn't actually have a triangle drawn into it, but it's readily perceived by the viewer. In fact, I'm betting you can't "not" see it.

The next image is a bit more of a challenge. What do you see when you carefully examine the image below? Hint: There could be 101 spots in this image, though I haven't counted.)

Did you find the Dalmatian? Images like these are closely related to camouflage art, which will be examined in part three of this series.

Illusions created by what we do not see can occur when clever engineering disguises working elements of a design. Infinity pools incorporate this technique to create stunning landscape effects. A careful look will show you where the man-made swimming pool ends and the natural landscape begins in this beguiling photograph by Margory.June. (Used by CC Generic 2.0 license.)

Spain and Switzerland boast sculptures that use simple-yet-ingenious engineering techniques to create a visual illusion. The plumbing that provides the water for this Zurich faucet is hidden inside the falling water cascades. The hidden pipes provide support for the faucet itself, encouraging the viewer to believe the faucet is floating in thin air.

Be sure to scroll through the photos below for more optical illusions found in everyday objects if you just keep an "eye" out for them!

Photo Gallery

The scintillating grid effect has been captured in this artworked, photographed by Andreia Bohner. Source: Andreia, Flickr.
The scintillating grid effect has been captured in this artworked, photographed by Andreia Bohner. Source: Andreia, Flickr.
Brandon Atkinson illustrates that illusions take place even in simple objects like traffic cones. Is the cone pointing toward or away from you? Source: Atkinson000, Flickr.
Brandon Atkinson illustrates that illusions take place even in simple objects like traffic cones. Is the cone pointing toward or away from you? Source: Atkinson000, Flickr.
The honeycomb screen covered the train window uniformly, says photographer David Goehring, who titled his photo "The Light Barrier." Source: CarbonNYC, Flickr.
The honeycomb screen covered the train window uniformly, says photographer David Goehring, who titled his photo "The Light Barrier." Source: CarbonNYC, Flickr.
The shutter slats appear curved when looking at this photo, but artist Jan Tik noted another interesting effect taking place when you scroll up and down over the photo, too. Source: jantik, Flickr.
The shutter slats appear curved when looking at this photo, but artist Jan Tik noted another interesting effect taking place when you scroll up and down over the photo, too. Source: jantik, Flickr.
Which color is on top of the other? K. Kendall used the Necker effect to produce this image. Source: KKendall, Flickr.
Which color is on top of the other? K. Kendall used the Necker effect to produce this image. Source: KKendall, Flickr.
Lars Plougmann noticed the way the car park's floors appear crooked, even though the walls are straight and parallel. Source: Lars Ploughmann, Flickr.
Lars Plougmann noticed the way the car park's floors appear crooked, even though the walls are straight and parallel. Source: Lars Ploughmann, Flickr.
Luis Ragerich captured this sight-based illusion with his camera. Source: lragerich, Flickr.
Luis Ragerich captured this sight-based illusion with his camera. Source: lragerich, Flickr.
This photo of a building, by Matthew Juran, illustrates another Necker effect. It shows an optical effect of the windows. Source: Matthew Juran, Flickr.
This photo of a building, by Matthew Juran, illustrates another Necker effect. It shows an optical effect of the windows. Source: Matthew Juran, Flickr.
Do the lines in this original work slope or not? Source: Pizzodisevo, Flickr.
Do the lines in this original work slope or not? Source: Pizzodisevo, Flickr.
Pizzodisevo also created this original illusion using fractals and CAD software. Notice how it appears to move as your eyes move over it. Source: Pizzodisevo, Flickr.
Pizzodisevo also created this original illusion using fractals and CAD software. Notice how it appears to move as your eyes move over it. Source: Pizzodisevo, Flickr.

More about illusions

Some illusions work because of our psychology and what we expect, and our psychological makeup can affect what we see. Learn how the medical field and marketers are using this information, and check out more great illusions at Perception & Optical Illusions.

Still more illusions - and info on how they've made at least one of our everyday activities possible - can be found on my Perceptions page.

What's Your Point of View?

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    • KathyBatesel profile image
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      KathyBatesel 3 years ago

      Thanks, CherylsArt!

    • CherylsArt profile image

      Cheryl Paton 3 years ago from West Virginia

      It was interesting looking at all these optical illusions, thanks for the info.

    • aaxiaa lm profile image

      aaxiaa lm 3 years ago

      Awesome lens!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      Haha, Dean! Hopefully not too long. I'd hate to feel I'd been responsible for you going cross-eyed or something!

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      dean_w 5 years ago

      That was fun.....but how long until my eyes can see straight again?

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @cmadden: Thank you, CMadden.

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      cmadden 5 years ago

      Fun lens!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @Natashalh LM: Wow, thank you for stopping by!

    • Natashalh LM profile image

      Natashalh LM 5 years ago

      This is so cool! Thanks for the awesome pictures and explanations. I just can't get over some of these things.

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @pcgamehardware: Thank you, Big Joe! I truly appreciate your SquidAngel blessing, too.

    • pcgamehardware profile image

      pcgamehardware 5 years ago

      Totally cool, very informative and entertaining as well. Nice lens.

    • Deadicated LM profile image

      Deadicated LM 5 years ago

      Love this Lens!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @anonymous: I agree, though I imagine it's great for his job security when companies hire him!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @anonymous: I agree, but I imagine it's great for his job security if people hire him to do those works!

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      anonymous 5 years ago

      Beever's work is amazing. So much work that just must get rained away.

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @anonymous: Thanks, cedembeck! That's one of my favorites, too.

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      anonymous 5 years ago

      Especially liked he negative picture of woman and how I saw her normal in the white space. Excellent work!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @anonymous: It made me dizzy when I saw it, but I love it!

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      anonymous 5 years ago

      make me confused. the black and white circle is hypnotize.

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @teamunited12: You're welcome. And thank you, too!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @Countryluthier: Thank you!

    • KathyBatesel profile image
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      KathyBatesel 5 years ago

      @neuvee: Thanks, Neuvee!

    • neuvee profile image

      neuvee 5 years ago

      wow, i like this kind of eye illusions. nice share!

    • Countryluthier profile image

      E L Seaton 5 years ago from Virginia

      Nicely done, the eyes have it! Thanks for sharing a fascinatingly deceptive lens. Good on ya!

    • teamunited12 profile image

      teamunited12 5 years ago

      Howdy and thanks for visiting my page....Love that car eye trick.