Types of Optical Illusions: Classic Examples and Why We See Them

Why do optical illusions happen?

An optical illusion is when what you perceive differs from the actual shape, color or movement. It doesn’t happen because of poor eyesight; under the right conditions, it happens the same way to almost everyone.

Behind it is the fact that the information reaching the eye is always incomplete. The retina receives a flat image, and brightness changes with the surrounding light. To infer three dimensions and true colors from that, the brain builds an answer from relationships with the surroundings. An illusion is the gap that appears when this inference kicks in under different conditions.

• Judging by comparison with the surroundings: the same size looks bigger when surrounded by smaller things
• Subtracting the lighting: colors in shadow are interpreted as brighter than they really are
• Filling in missing information: broken lines are read as continuous lines that are just hidden
• Getting used to strong stimuli: staring at the same color or movement lowers sensitivity to it

Below, we look at the classic illusions in four groups: shape, brightness and color, motion, and interpretation. Organizing them by which mechanism is at work is easier to understand than memorizing names.

When shapes or lengths look different (geometric illusions)

This is the type where lengths, sizes or angles look different even though a ruler says they’re the same. It’s the largest group and the one most often featured in textbooks.

What they share is that the brain judges size not as “size = size on the retina” but as “size = size on the retina × estimated distance”. Put distance cues into the picture and that correction fires in the wrong direction.

When brightness or color looks different

This is the type where areas painted the same color look different, or where color appears in places with none at all. It’s also the group that causes the most real problems in image editing and design work.

In practical design work, the effect of simultaneous contrast matters most. The same gray text reads differently when the background brightness changes. When choosing colors, don’t compare them on their own; judge them on the background you’ll actually use.

When still images seem to move (motion illusions)

This is the type where a still image seems to rotate slowly or ripple. It’s strongest in peripheral vision, and the movement stops when you stare at one point.

• Rotating snakes illusion: concentric bands of color seem to turn every time you move your eyes
• Fraser-Wilcox type: the order of light and dark misleads the motion-detection system in one direction
• Motion aftereffect (waterfall illusion): after watching movement in one direction, a still image seems to move the opposite way
• Beta movement: switching between still images in slightly different positions feels like motion. The principle behind animation and video

The motion aftereffect is explained by the same kind of adaptation as color afterimages. When the system that responds to downward motion tires, the upward response becomes relatively stronger when you look at a still object, so it seems to move the opposite way. It’s exactly the same setup as seeing the opposite color in a color afterimage.

When interpretation flips or you see things that aren’t there

This type happens when a picture lacks information or allows more than one interpretation. What you see may flip partway through, or you may see outlines that weren’t drawn.

This group works more strongly the less information there is. When you draw a picture in a few cells, as in pixel art, it’s really just a collection of coarse dots, yet you recognize it as one shape with a continuous outline. It gets across with so little information because the brain fills in the gaps.

Try them yourself

Illusions stick in your memory better when you experience them than when you read about them. Start with the ones you can do right away.

1. 1

   Experience a complementary afterimage

   In the color afterimage tool, stare at the inverted image for 20 seconds, then switch to black and white. Comparing the colors you saw with the original color image shows what your eyes were doing.

2. 2

   Check simultaneous contrast

   In an image editor, make two squares of the same gray and put one on a white background and the other on black. They won’t look like the same color.

3. 3

   Draw a shape with little information

   Try drawing a picture on a grid of about 10 × 10 cells in pixel art. Testing how far you can reduce it and still get the shape across lets you feel the filling-in at work.

4. 4

   Try the motion aftereffect

   Watch scrolling text for about 30 seconds, then move your eyes to still text, and it seems to flow the other way.

Applying them to design and presentations

Illusions aren’t just fun material; they’re also knowledge for designing how things look. Knowing them helps you avoid unintended distortions.

• Choose colors together with their background: because of simultaneous contrast, a color picked on its own looks different on the actual screen
• Fine-tune shape sizes by eye: a circle looks smaller than a square of the same height, so make circular icons slightly larger
• Charts get misread by area: doubling the radius quadruples the area, exaggerating the real difference
• Avoid fine stripes: evenly spaced thin lines can cause flicker or false colors
• Don’t rely on color alone: also using brightness contrast or differences in shape makes things readable for everyone

In charts and diagrams especially, it happens that “I drew the correct numbers, but it was misunderstood”. Checking how viewers will actually take it is how someone who knows about illusions works.

FAQ

Why do optical illusions happen?

Because the information reaching the eye alone can’t pin down shape, color or distance. The brain infers an answer from relationships with the surroundings and past experience, and that decides what you see. The inference usually works accurately, but when cues for it are planted in a picture, it goes the wrong way and you see something different from reality. That’s an optical illusion.

What types of optical illusions are there?

Broadly, there are four: geometric illusions, where lengths or sizes look different; brightness and color illusions; motion illusions, where still images seem to move; and cognitive illusions, where interpretation flips or you see outlines that aren’t drawn. The Müller-Lyer illusion belongs to the first, the checker shadow illusion and complementary afterimages to the second, the rotating snakes illusion to the third, and Rubin’s vase to the fourth.

I can’t see the illusion. Is something wrong with my eyes?

No. People see illusions differently; with the same picture, some see it strongly and others barely at all. It also changes with viewing distance, screen brightness and where you look. Motion illusions in particular are strongest in peripheral vision, so the movement stops if you stare at one point. Stepping back and looking at the whole picture may help.

Can I make an optical illusion myself?

Yes. For the color afterimage illusion, all you need is an image of one of your photos inverted to its complementary colors and a black-and-white version. There’s a tool that inverts and desaturates in the browser automatically and lets you save the inverted image as a PNG, so you can use it to make materials for class or presentations.