Optical Illusions: Sight-based illusions
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.
MoirÃ© 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 moirÃ© effect if you slid one over the other. This cone uses a moirÃ© 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.
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...
Make Your Own Afterimage Photos on iPhone
- iPhone After Image Ap
This is a free ap that users say works well enough, but that it quickly loses interest for them.
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.
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.
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!
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.