How do optical illusions work in designs?

As a child, I trained to paint in a style called Trompe-l’œil or to trick the eye. Using this technique the artist attempts to paint so realistically that the viewer’s mind is fooled into believing that the objects or scenes depicted in the painting are three-dimensional objects.

The Old Violin, by William Harnett (1886)

The Old Violin, by William Harnett (1886)

I was endlessly fascinated with paintings like The Old Violin and sought to understand how our eyes can trick the brain into perceiving dimensionality that isn’t there.

When you look at a design or painting, your eyes are trying to make sense of the light that has bounced off the picture and onto your retina. The pigments on the image absorb the different wavelengths of light at different rates. White pigment barely absorbs any light at all and lets much of the light that hits it bounce off. Blue pigment absorbs the majority of longer wavelengths as blue is one of the shortest wavelengths we can see.

As the information that is picked up by our eyes is only light, our individual retinas don’t have the ability to tell us if something is 3D or not. Working together our eyes get some depth information because the light that hits one eye is slightly offset compared to the other eye.

Then our mind merges the light perceived by both eyes to form the perception of three dimensions.

However! Because our eyes are only offset about 2.5 inches, stereo vision starts to fail for objects that are about 8 feet away. This means that far away objects are giving you little depth information from stereo vision.

You can see this for yourself by extending your arm out with your thumb up. Focus your eyes on your thumb, then on something in the background behind it. Next, close one eye and look at your thumb, then the background. Notice how much the image of your thumb moves as you close each eye.  Now do this with an object in the background. The item in the background moves less if at all.

Retinal disparity

Since our eyes aren’t so hot at discerning depth cues at farther distances, we use other tools to understand dimensionality.

One the easiest to use in art and design is occlusion. Our brain assumes that when one object obscures another that it is in front of the other. For example in the image below what do you see?

Most people see a white triangle covering 3 circles and another triangle. Our brain spends our entire lives trying to make sense of the world and creates rules of thumb based on previous experiences. Often we see items in front of other objects and our mind assumes that this is the case for the triangle example.

The above image is also working by fooling our mind based on a large topic in neuroscience named Gestalt principles (which will be the topic of future posts). In short, Gestalt principles describe the rules our minds use to combine the simple information that

we see into complex scenes. In the triangle example, Gestalt theory suggests that our minds tend to (by default) think of complete objects – if we see two parts of a line our mind perceives that it is a full line with some section blocked off.

This is a fun principal to use in design and art because much of the work is being done in the viewer’s head rather than in the image.

The effects of our brains “making sense” of the world are strongest for ambiguous image optical illusions. In these illusions you can see the image as a few different kinds of objects, but once you see it one way it is hard to switch to see the other. Once your mind has made sense of the image you have to actively work to make it see a scene differently.

Graphic Design Optical Illusion

Ambiguous Image Optical Illusion

There are two other types of optical illusions, those that seem to move and those that seem to include shapes that aren’t there.

The illusions in the moving category are fun because they work on cats but not newborns.

Cats’ visual systems, like our own, perceive lighter values more quickly than dark values which make an image with alternating light and dark areas seem to move. However, newborns’ eyes are not developed enough to see these illusions. Feel free to test this on your baby!

Finally, there are illusions that seem to show shapes or colors that aren’t even there. These illusions work by messing with the cells in your eyes. Cells can get ‘tired out by high contrast images and perform the opposite action. For example, stare at the center of the flag below for 30 seconds then look at a white wall or white piece of paper.


American flag optical illusion

If you saw a normal red, white, and blue flag on the white wall your cells were tired out by the high contrast and colors of the blue, black and yellow flag. These cells were overpowered by cells that absorb their opposing colors, creating an afterimage with the associated colors in the cell.


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