ArchiveOriginally published 2009
20 Optical Illusions That Will Have You Stumped (Rebuilt)
The most shared post in this site's history was a gallery of twenty optical illusions. The original images are gone, so here they are again, drawn fresh, with a short explanation of what your visual system is doing when each one fools you.
On this page
1. Muller-Lyer2. Hermann grid3. Ebbinghaus4. Kanizsa triangle5. Cafe wall6. Zollner7. Ponzo8. HeringTwelve more from the original listWhy illusions matter beyond funCommon questions1. Muller-Lyer
The line with outward arrowheads looks longer. One explanation is that the fins look like the corners of a room seen from inside (far) or outside (near), and the brain scales the line accordingly.
2. Hermann grid
Cells in the retina compare a spot with its surroundings. At an intersection there is more white around the spot than along a bar, so the intersection is signalled as slightly darker.
3. Ebbinghaus
A circle surrounded by large circles looks smaller than the same circle surrounded by small ones. Size is judged relative to neighbours, not in absolute terms.
4. Kanizsa triangle
The visual system completes contours across gaps. The three notched discs are read as circles partly hidden by a triangle, so a triangle is created to explain them.
5. Cafe wall
Offset rows of alternating tiles make the mortar lines appear to slope. Small brightness differences at the tile edges are integrated into a sense of tilt.
6. Zollner
Short cross-hatches at alternating angles make the long lines appear to converge and diverge. The brain exaggerates the angle between nearby lines.
7. Ponzo
Between converging lines that read as receding railway tracks, the upper bar is interpreted as farther away and therefore larger.
8. Hering
Radiating lines behind two verticals make them bow outward. The illusion is thought to come from the brain over-estimating the angles where lines cross.
Twelve more from the original list
| Illusion | What you see | What is really there |
|---|---|---|
| Checker shadow | One square in shadow looks lighter than one in light | Both squares are the same grey |
| Rotating snakes | Concentric rings appear to rotate | A static image; motion comes from eye movements and contrast timing |
| Necker cube | A wire cube flips between two orientations | A flat line drawing with no depth cue |
| Rubin vase | A vase, or two faces in profile | One outline shared by both readings |
| Spinning dancer | A silhouette spins clockwise or anticlockwise | An ambiguous 2D animation |
| Lilac chaser | A green dot circles, then the lilac dots vanish | Only lilac dots blinking in sequence |
| Scintillating grid | Black dots flash in white discs | Static grey lines with white discs |
| Fraser spiral | A spiral | Concentric circles over a twisted-cord texture |
| Shepard tables | Two tabletops of different shape | Identical parallelograms |
| Jastrow | Two curved shapes of different size | Identical shapes, one placed below the other |
| Troxler fading | A blurred spot disappears when you stare | Constant stimulus; the retina adapts |
| Moon illusion | The moon looks huge near the horizon | Same angular size as overhead |
Why illusions matter beyond fun
Every illusion above marks a shortcut the visual system takes because it usually works. Judging size by context is how you know a distant person is not tiny; completing contours is how you recognise a cat behind a fence. Vision researchers use these failures to map how the shortcuts are built, and interface designers use the same knowledge in reverse: a progress bar that appears to move faster near the end, or a button that looks raised because of a one-pixel highlight, is an illusion doing useful work.
Common questions
Why do optical illusions work?
Because vision infers a 3D world from 2D input using assumptions about depth, lighting and context. A drawing that violates those assumptions produces a confident wrong perception.
Are optical illusions the same for everyone?
Mostly, though strength varies between people and some illusions are weaker in cultures with less exposure to carpentered, rectangular environments.
Can knowing the trick make an illusion stop working?
Rarely. Illusions happen in early visual processing that knowledge does not reach; you can measure the lines and still see them as unequal.
Are illusions bad for your eyes?
No. Some motion illusions can feel uncomfortable for a moment, but they cause no harm.
Where did the original twenty images go?
The original post's images were lost when the site went offline. The illusions here are redrawn as vector graphics so they stay sharp at any size.