The illusion file
Sourced facts about optical illusions and the devices that trick the eye. Every fact links to its source, checked 31 Oct 2026.
Size illusions
- The Müller-Lyer illusion, two equal lines that look unequal because of the arrowheads on their ends, dates from 1889. [source]
- In the Müller-Lyer illusion, the lines at the ends lead the eye inward or outward and create a false impression of length. [source]
- Try to mark the middle of a Müller-Lyer line and big errors are normal. Vision scientist Michael Bach says he is out by about 10%. [source]
- In the Ponzo illusion, a shape touching two converging lines looks bigger than an identical shape where the lines are further apart. [source]
- The Ebbinghaus illusion, where a circle ringed by small circles looks bigger than an identical one ringed by big circles, is also called the Titchener illusion. Titchener published it in a textbook in 1901. [source]
- The Ebbinghaus illusion is weaker in children under 10. [source]
- The moon looks larger near the horizon than high in the sky. [source]
- Test the moon illusion yourself: at arm’s length, about two-thirds of your little finger covers the moon, however high or low it is. [source]
- Shepard’s tables, two table tops that look completely different but are the same shape on the page, were created by the psychologist Roger N. Shepard. His original was called “Turning the Tables”. [source]
Impossible objects
- Impossible objects were formally described by the Penroses in a 1958 paper called “Impossible objects: a special type of illusion”. [source]
- Each corner of an impossible object makes sense on its own. The trouble is that the corners combine viewpoints that can’t all be true at once. [source]
- The “devil’s fork”, three prongs at one end and a U-shaped bar at the other, is also known as a blivet. [source]
- Artists drew impossible scenes long before the Penroses. William Hogarth’s Satire on False Perspective dates from 1754. [source]
Ambiguous figures
- In the famous vase-and-faces figure, you see either a white vase or two black profiles facing each other, and the brain keeps switching between them. [source]
- A Necker cube, a simple wire-frame cube, seems to flip between two orientations, typically every 3 to 5 seconds. [source]
- Try to “hold” one view of a Necker cube and you normally can’t: after 10 seconds or so it flips again. [source]
- Look at the inside of a hollow face mask and it flips back to looking like a normal face. You can’t make it stay hollow. [source]
Pareidolia
- Seeing faces or familiar shapes in random patterns, such as animals in clouds or rock formations, is called pareidolia. [source]
- In 1976 a Viking orbiter photographed a rock formation on Mars, in the Cydonia region, that looked like a face. In April 1998, after a public outcry, the Mars Global Surveyor was programmed to photograph it again. [source]
Stereograms and stereoscopes
- Stereoscopy, making flat pictures look solid by showing each eye a slightly different view, was first described in 1832 by the English physicist Sir Charles Wheatstone. Sir David Brewster improved it. [source]
- Stereoscopic photos (stereographs) were hugely popular in the United States and Europe from about the mid-1850s into the early 20th century. [source]
- Charles Wheatstone, inventor of the stereoscope, also invented the concertina, a small accordion. [source]
Camera obscura
- Camera obscura is Latin for “dark chamber”. The earliest ones were small dark rooms with a single tiny hole that cast an upside-down picture of the outside scene on the opposite wall. [source]
- By the 16th century, artists used the camera obscura as an aid to drawing, tracing the image cast on paper. [source]
- The camera obscura is the ancestor of the photographic camera. [source]
Moving pictures
- The zoetrope, a spinning drum lined with a changeable band of pictures, was invented by William George Horner around 1834. [source]
- In 1876 the Frenchman Émile Reynaud adapted the zoetrope idea so it could be projected for a theatre audience. [source]
- The phi phenomenon is the illusion of movement you get when lights placed side by side switch on rapidly one after another. [source]
- Shown at 20 frames a second, a spoked wheel turning at about 100 rotations a minute seems to stand still, and turn it faster and it can seem to go backwards. It’s the wagon-wheel effect from old westerns. [source]
Afterimages and fading
- An afterimage is an image that lingers after you stop looking at something. A negative afterimage shows the opposite, complementary colours. [source]
- The spot you see after a camera flash goes off is an everyday afterimage. [source]
- In the “lilac chaser”, a ring of magenta dots with one missing at a time, stare at the centre and you see a green dot racing round. Its inventor is Jeremy Hinton. [source]
- Stare beside a fuzzy-edged dot and it fades away. Your eyes are always making tiny jittery movements, and a soft edge gives them no new information. [source]
Colour and shade
- In Ted Adelson’s checker shadow illusion, a square in shadow and a square in the light look very different but are exactly the same shade. [source]
- Spin a black-and-white disc with the right pattern and you see faint colours. The toy maker Charles E. Benham sold one as an “Artificial Spectrum Top” in 1894. [source]
- In the Hermann grid (1870), dark patches appear where the white “streets” cross, except at the crossing you look straight at. [source]
- Everyone has a blind spot: where the optic nerve passes through the retina, there are no light receptors. Your brain fills in the gap, so you don’t notice it. [source]
Op art
- Op art is a branch of mid-20th-century geometric abstract art that deals with optical illusion. [source]
- Op art first caught international attention with “The Responsive Eye” exhibition at the Museum of Modern Art in New York in 1965. [source]
- Op art painters used simple repeated forms such as parallel lines, chequerboard patterns and concentric circles. [source]
- The café wall illusion was first reported by Richard Gregory and Priscilla Heard in 1979, after one of Gregory’s team noticed the tiled front of a café on St Michael’s Hill in Bristol. [source]
- In the café wall illusion the rows of tiles seem to slope, but the lines are actually parallel and every tile is square. [source]
Try it: find your blind spot
On a small card, draw a dot on the left and a cross on the right, a few centimetres apart. Hold the card at arm’s length, close your right eye and look at the cross with your left eye. Slowly bring the card towards your face. At one point the dot vanishes. [source]
Books and kits for illusion hunters
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For a rainy afternoon or a classroom: Optical illusion books · Illusion books for children · Stereogram books · Zoetrope kits · Pinhole camera kits · Kaleidoscopes · Optical illusion jigsaws. These are Amazon UK search links, not endorsements of particular products. As an Amazon Associate I earn from qualifying purchases.