94 Fun Facts About Optical Illusions

A 2015 viral photo split viewers over whether a dress was white and gold or which other pair of colours?

Blue and black was the real dress, yet the washed-out photo made many people see white and gold; a 2015 survey found that answer was more common among older viewers and women.

Which class of illusion is thought to come from overstimulating the eyes or brain, like the spots left by a camera flash?

Physiological illusions are thought to come from excessive or repetitive stimulation of one neural channel early in visual processing, which leaves an imbalance that distorts what is seen next.

Impossible objects like the Penrose triangle are which kind of cognitive illusion?

Paradoxes are pictures of objects that could never exist: each corner of the drawing makes sense by itself, but the eye cannot join them into one solid shape.

A stick appearing bent in water is the classic example of which class of illusion?

The bent stick is a physical illusion: it comes from light itself, which changes direction as it leaves the water before it ever reaches the eye, not from anything the eye or brain does.

Rubin's famous reversible figure can be seen as two faces in profile or as which object between them?

A vase: Danish psychologist Edgar Rubin published the figure in 1915 to show that the brain must pick one shape as the object and treat the rest as background, so you see faces or vase, never both.

In the Müller-Lyer illusion, the orientation of what at the shafts' ends makes equal lines look different?

The fins can point inwards to form an arrow head or outwards to form a tail, and the two configurations are judged unequal even by viewers who know the trick.

A 1963 study linked the Müller-Lyer illusion to growing up among what?

Right-angled buildings: Segall, Campbell and Herskovits called such surroundings 'carpentered', arguing that eyes raised among straight edges and square corners learn to read the arrow shapes as corners seen in depth.

The Moon illusion makes the Moon look largest when it is in which position?

Near the horizon: photographs prove the Moon is the same width there as when it rides high, and after thousands of years of watching it there is still no agreed explanation.

One theory explains the Müller-Lyer illusion as the brain misapplying which perceptual constancy?

Gregory's theory blames misapplied size constancy: the arrow ends act like perspective depth cues, so the mechanism that keeps a receding person from looking smaller misjudges a flat line's length.

In the Müller-Lyer figure, how does the shaft between two arrow 'tails' look compared with the one between 'heads'?

A 2005 analysis of natural images found that the tail-type figure really does tend to come from physically longer edges in a scene, which may be why the brain guesses as it does.

Hermann Ebbinghaus, namesake of the circles illusion, pioneered the experimental study of what?

Memory was his field: inspired by a second-hand copy of Fechner's Elements of Psychophysics, Ebbinghaus tested himself with lists of nonsense syllables and discovered the forgetting curve.

In the Ponzo illusion, two identical bars cross a pair of lines converging towards the top; what do you see?

The upper looks longer: on one prominent account, Mario Ponzo's converging lines read as distance, so the brain treats the upper bar as farther away and scales it up.

In the Ebbinghaus illusion, a central circle looks undersized when surrounded by what kind of circles?

Larger circles make the centre look smaller, and distance matters too: the closer the surrounding ring sits, the bigger the central circle appears.

The Ebbinghaus illusion has been found to deceive which group less often than university students?

Doherty and colleagues reported in 2010 that children aged 10 and under were deceived less than university students, attributing the adults' greater susceptibility to higher context sensitivity.

Which 1960s art movement, led by Bridget Riley and Victor Vasarely, used geometric patterns that seem to vibrate and move?

Op art, short for optical art, caught the public's eye when New York's Museum of Modern Art staged 'The Responsive Eye' in 1965, after which its patterns were borrowed for clothing and other commercial designs.

Which Dutch artist's 1961 lithograph 'Waterfall' sends water around a circuit built from Penrose triangles?

M.C. Escher's Waterfall runs a zigzag watercourse along Penrose triangles so that it ends higher than it began; his earlier prints had partly inspired the Penroses to devise the triangle.

What is the term for seeing faces in clouds, car fronts or the surface of the Moon?

Pareidolia made a Martian mesa famous: in a 1976 Viking 1 photo, shadows gave it eyes, a nose and a mouth, and it became known as the 'Face on Mars'.

What was the profession of Louis Albert Necker, who first published his wire-frame cube in 1832?

Louis Albert Necker was a Swiss crystallographer who reported the cube's spontaneous reversal in 1832 after noticing engraved drawings of crystals seeming to change orientation.

Staring at a Necker cube, the wire-frame drawing with no depth cues, your sense of its orientation tends to do what?

Flip back and forth: every line of the drawing is ambiguous, so it fits two differently oriented cubes equally well and the brain keeps switching between them with no change on the page.

Why don't you normally notice the blind spot where the optic nerve leaves each retina?

The brain fills it in: there are no light receptors where the optic nerve exits, so the brain extends the surrounding scene across the gap, and the other eye covers the same patch.

Which object is hidden as a stretched smear across the floor of Holbein's 1533 painting 'The Ambassadors'?

A skull: Holbein painted it in distorted perspective, called anamorphosis, so the long shape between the two men's feet turns into a skull only when the picture is seen from a sharp angle.

An Ames room, a distorted room viewed through a peephole, makes people appear to do what as they walk between its corners?

Grow or shrink: from the peephole the slanted room looks square, and because square rooms are the norm the brain accepts that, so the people crossing it seem to swell or dwindle instead.

The Ames room illusion is viewed with one eye through a peephole, to remove depth cues from what?

Stereopsis, which needs two eyes; the room's geometry is worked out in perspective so that, from the peephole alone, the retinal image matches an ordinary cuboid room.

The true shape of an Ames room is typically that of an irregular what?

The room is an irregular hexahedron: every face can be a regular or irregular quadrilateral, so one corner ends up much farther from the viewer than the other.

Which film trilogy used forced-perspective sets to make some characters look far shorter than others in the same shot?

Peter Jackson's The Lord of the Rings trilogy placed actors at different distances from the camera on distorted sets, so hobbits looked far shorter than Gandalf in the same shot.

A Fata Morgana, with its rapidly changing stack of images, is an elaborate form of which kind of mirage?

A Fata Morgana is a superior mirage: under a strong temperature inversion, distant ships or coasts appear to float, stretched and stacked in alternating upright and inverted images.

Mirages are produced by which optical phenomenon?

Refraction: light bends as it crosses layers of air at different temperatures, so distant objects appear displaced; unlike a hallucination, a mirage can be photographed.

Fata Morgana mirages took their name from sightings over which strait?

Fata Morgana is Italian for the sorceress Morgan le Fay, blamed for the castles seen floating over the Strait of Messina between Sicily and Calabria.

'Mirage' comes from the Latin 'mirari', also the root of 'miracle' and 'admire', meaning what?

To wonder at: on the usual account, French turned 'mirari' into 'se mirer', to look at oneself or be reflected, which gave 'mirage' and also 'mirror', so the word drifted from marvelling to mere appearance.

Which French term names paintings so lifelike that viewers mistake a painted door or curtain for the real thing?

Trompe-l'œil is French for 'deceive the eye': in an ancient tale, Zeuxis painted grapes so real that birds flew at them, yet he was fooled himself by a curtain that his rival Parrhasius had painted.

Which English scientist, inventor of the concertina, built the first stereoscopes in the 1830s?

Charles Wheatstone presented his mirror stereoscope to the Royal Society in June 1838, a year before the first practical photographic process, so his earliest stereoscopes displayed drawings.

Which 1990s book series hid 3D pictures inside repeating patterns that readers had to stare 'through'?

Magic Eye books are autostereograms: engineer Tom Baccei and artist Cheri Smith made the first full-colour ones in 1991, improving on black-and-white random-dot pictures invented for vision research.

What is the usual way to view an autostereogram, focusing 'through' the image?

View a wall-eyed image cross-eyed and the hidden scene appears as a cut-out sunk behind the background instead of popping out.

Which effect makes straight mortar lines between staggered dark and light tiles look sloped?

The café wall illusion is named after a real one: Richard Gregory's Bristol lab noticed it on the wall of a café on St Michael's Hill, though the pattern had been described decades earlier.

The rabbit–duck drawing debuted in 1892 in Fliegende Blätter, a humour magazine from where?

Germany: the humour weekly printed the drawing unsigned in 1892, and American psychologist Joseph Jastrow then used it to show that seeing depends on the mind as well as the eye.

Stare at a bright green patch, then look at a white wall: what colour is the afterimage you see?

Magenta: the cone cells tuned to green tire and respond less, so white light then stimulates mostly the red and blue cones, and the brain reads that mix as magenta.

What does the Hermann Grid illusion make you see at the intersections of a white grid on black?

Ludimar Hermann reported the grey blobs in 1870; they appear at intersections seen in peripheral vision and vanish the moment you look directly at one.

In the Hering illusion, what do parallel lines over radiating spokes seem to do?

Bow outwards: Ewald Hering described the effect in 1861, and one account says the spokes mimic perspective, giving a false impression of depth that bends the lines.

What term names the apparent wandering of a fixed point of light in a dark, featureless setting, like a lone star?

The autokinetic effect: with no background to anchor it, a single steady light in the dark seems to drift, probably because the eye judges movement against reference points and here there are none.

Kanizsa's triangle is induced by three of which shape, each with a wedge removed?

Three circles, each missing a wedge like a Pac-Man, are aligned so the gaps line up, and viewers see a bright triangle whose edges are not on the page; Gaetano Kanizsa published it in 1955.

On a hot day, the shimmering 'puddle' on the road ahead is really a mirage of what?

The sky: a layer of very hot air just above the tarmac sends light from the sky up to your eye, so the brain reads the bright patch on the ground as a pool of water.

In the Poggendorff illusion, how does a diagonal line passing behind a bar look?

Misaligned: Johann Poggendorff, a journal editor, spotted the effect in 1860 in a drawing that the astrophysicist Zöllner had sent him to show a different illusion.

Patients with which psychiatric condition are notably less fooled by the hollow-mask illusion?

Schizophrenia patients see the hollow mask as hollow far more often than controls; a 2009 imaging study linked this to weaker top-down processing, which normally overrides the depth cues.

The Barberpole illusion reveals biases in the brain's processing of what kind of information?

The barber pole illusion concerns motion: as the pole spins its diagonal stripes physically move sideways, yet they are seen rising, because the aperture problem biases the perceived direction.

Which British prime minister's upside-down photo, its eyes and mouth flipped, named a famous face illusion?

Margaret Thatcher was in office when psychologist Peter Thompson doctored her portrait in 1980. Upside down it looks fine; turn it upright and the changes become glaring.

Which Victorian theatre trick with angled glass put a virtual Tupac Shakur on a concert stage in the 2010s?

Pepper's ghost takes its name from John Henry Pepper, who used it to haunt a Dickens play in London in 1862. The audience sees a hidden, brightly lit figure reflected in a pane they never notice.

Which film shows the Penrose stairs, a loop of steps that climbs forever, as a trick of dream design?

Inception can build the staircase only inside its dream worlds, where a few turns always lead back to the start. Lionel and Roger Penrose, father and son, made the design famous in the 1950s.

In a 1937 Salvador Dalí double image, swans on a lake are mirrored in the water as which animals?

Elephants appear because the swans' necks turn into trunks, their bodies into ears and the bare trees behind them into legs. Dalí filled his 1930s canvases with such double images.

Which legendary phantom vessel, doomed to sail forever, may owe its origin to Fata Morgana mirages seen at sea?

The Flying Dutchman is said to glow as it sails where no vessel should be. A Fata Morgana can stretch, multiply and even invert the image of a real one far away.

A famous hybrid image shows Albert Einstein up close, but whose face emerges when the viewer steps back?

Marilyn Monroe sits in the picture's coarse, blurry detail, which takes over at a distance, while Einstein is drawn in the fine lines. Squinting brings her out too.

The lilac chaser illusion, a gap racing round a ring of blurred dots, is also named after which arcade game?

Pac-Man lends the illusion its other name because a moving spot seems to gobble the discs one by one. The spot is not on screen at all: it is an afterimage left by each missing disc.

At a 'gravity hill', the surrounding land fools the eye so a car left in neutral appears to do what?

Cars seem to roll uphill because the road really slopes gently down. With the true skyline hidden, leaning trees and banks hand the eye a false level, and locals often blame magnetism.

Dazzle camouflage, the clashing stripes and shapes of the First World War, was painted on what?

Ships wore dazzle not to hide but to make an enemy misjudge their speed and heading. Its champion was a marine painter, Norman Wilkinson, and Picasso claimed the Cubists had invented it.

Two identical photos of which landmark, set side by side, won a 2007 illusion contest by seeming to differ?

Tower of Pisa photos, shot looking up, appear as if the right-hand one leans more. The eye expects two real towers to converge with distance, so parallel copies seem to splay apart.

In an ancient Greek contest, birds are said to have pecked at the painter Zeuxis's lifelike picture of which fruit?

Grapes won Zeuxis the birds but lost him the contest. His rival Parrhasius then invited him to pull back a curtain, and the curtain turned out to be the painting.

A 1915 Puck magazine cartoon by W. E. Hill reads as a young wife looking away or as which other figure?

An old woman appears when the young one's chin becomes a nose and her choker a mouth. Hill called the cartoon 'My Wife and My Mother-in-Law', though an 1888 postcard had used the idea first.

Which humour magazine introduced the 'Poiuyt', a three-pronged fork that could never exist, on a March 1965 cover?

Mad named the fork by reading the top row of a typewriter keyboard backwards. Three round prongs somehow turn into two square ones along the way, so nobody can build the thing.

Classic paper 3D glasses fuse two overlapping pictures into one deep scene, most often with lenses of which two colours?

Red and cyan lenses each block one of the two overlapping pictures, so each eye gets its own view and the mind fuses them into depth. Fold the two lenses together and everything goes dark.

After a minute staring at a waterfall, the still rocks beside it briefly appear to drift in which direction?

Upwards is the way they drift, because nerve cells tuned to falling water tire and go quiet, tipping the balance the other way. Robert Addams described it in 1834 at the Falls of Foyers in Scotland.

The Spinning Dancer, a silhouette that twirls either way, went viral in 2007 wrongly billed as a test of what?

Brain dominance was the pitch: each direction of spin supposedly marked a 'right-brained' or 'left-brained' viewer. No such link exists, and a flat silhouette simply gives no clue to near and far.

Edward Adelson's 1995 illusion, where squares A and B look dark and light yet match exactly, is drawn on what?

A checkerboard lets the trick work: square B lies in a cast shadow, so the mind 'corrects' for the gloom and reports it as pale. Cover everything else and the two greys are identical.

Stare hard at one point and unchanging shapes at the edge of vision do what, as Ignaz Troxler noted in 1804?

They fade away because nerve cells stop reporting a signal that never changes, just as skin soon stops feeling a scrap of paper resting on the arm.

The Novaya Zemlya effect, a polar mirage first recorded by Dutch explorers in 1597, shows what about two weeks early?

The Sun appeared to Gerrit de Veer, iced in for the polar night, a fortnight before it was due back. His crewmates accused him of counting the days by the wrong calendar.

Which planet's 'canals', mapped by astronomers such as Percival Lowell, proved to be an optical illusion?

Mars gained its canals partly through mistranslation: Giovanni Schiaparelli wrote 'canali', Italian for channels, and English readers took them for waterways built by engineers.

Tilt-shift photography blurs parts of a picture of a real city so that the scene resembles what?

A scale model seems to appear because close-up photos have a very thin zone of focus, so blur tells the eye the scene is tiny. Shooting from high up and speeding up video make it stronger still.

Which 16th-century Italian painter built portrait heads entirely out of fruit, vegetables, flowers and fish?

Arcimboldo was court portraitist to three Habsburg emperors, and his heads of fruit, roots and sea creatures were painted partly as curiosities to amuse the court.

Short of funds at Rome's church of Sant'Ignazio, Andrea Pozzo painted a flat canvas in 1685 to imitate what?

A dome is what visitors see from a marker set in the nave floor. The 1685 original was destroyed by fire, and today's canvas is a copy made in 1823 from Pozzo's drawings.

The Jastrow illusion, a magic-shop favourite, is often shown with two identical curved pieces of which toy?

Railway track curves show it neatly: lay two identical pieces one above the other and the lower one seems bigger. Swap them over and the 'bigger' piece changes.

In James Fraser's 1908 'twisted cord' illusion, a set of separate concentric rings appears to form which shape?

A spiral is what the eye insists on, yet a finger traced along any arc comes back to where it began. The tilted strands and patterned background supply twists that are not there.

Which physicist, whose name also measures supersonic speed, described false stripes where grey shades meet in 1865?

Ernst Mach saw that the eye exaggerates edges, adding a bright stripe and a dark one that no light meter records. He proposed that cells in the retina damp down their neighbours.

In Roger Shepard's 1990 book Mind Sights, two identical parallelograms look wildly different when drawn as what?

Tabletops work because the eye reads them in 3D and stretches whichever side seems to recede. Viewers misjudge the lengths by 20–25%, and Shepard found that knowing the trick barely helps.

Which 1820s optical toy is a disc on strings that, when twirled, merges the pictures on its two sides?

Thaumatrope discs often put a bird on one side and a cage on the other, so twirling the strings shut the bird inside. Many also carried a riddle, one line per side.

Max Wertheimer's 1912 study of two flashing lights seen as one moving light launched which school of psychology?

Gestalt thinking grew from his point that viewers see movement directly rather than working it out from two separate flashes. Wertheimer named the effect the phi phenomenon.

Benham's top, an 1895 toy disc printed only in black and white, makes viewers see what when it spins?

Pale colours shimmer in arcs across the spinning disc, although its ink is only black. Toymaker Charles Benham sold it to bring an effect Gustav Fechner had demonstrated to English speakers.

After 15 minutes of staring at coloured stripes, the McCollough effect has been recorded lasting how long?

Nearly three months is the record, set by volunteers who were not retested in between. Those checked every few days lost the tinted stripes within five, as if looking were what erased them.

In the Pulfrich effect, a dark filter over one eye makes a pendulum swinging side to side appear to trace what?

An ellipse appears because the dimmed eye reports a split second late, and the mind reads that lag as depth. A sunglass lens held over one eye is enough to try it.

Which mountain apparition is a walker's own giant shadow thrown onto mist, often ringed by a rainbow halo?

Brocken spectre sightings take their name from the foggy highest peak of the Harz range, where the shadow fed a local legend. It seems enormous because mist offers nothing to judge its distance by.

Which Greek term names the slight swelling of a temple column, long said to stop its sides looking pinched inward?

Entasis was explained by Hero of Alexandria as a cure for columns that would otherwise seem to cave in. Modern research suggests viewers see no such illusion, so the swell may simply signal strength.

Whose figure, sketched by an astrophysicist in 1860, makes long parallel lines look tilted where short strokes cross?

Zöllner posted the sketch to a journal editor, who spotted a second illusion hiding in the same drawing. That one now carries the editor's name.

When cars in the next lane creep forward and a driver feels a stopped car rolling backwards, which illusion is at work?

Vection strikes when the mind takes movement filling the edge of vision as its own. Pilots are warned about it, because the same thing can happen while taxiing an aircraft.

In the 'stopped-clock illusion', a second hand seems to freeze for a moment just after a viewer does what?

A viewer who first glances at it has just made a rapid eye jump, and the mind backdates the new image to cover the blur. That opening tick can seem up to half a second too long.

Which English artist filled his 1754 engraving 'Satire on False Perspective' with deliberate errors of depth and scale?

William Hogarth showed a man on a far hill lighting his pipe from a candle held out of a nearby window. The caption warned that designers ignorant of perspective would commit such absurdities.

Which New York institution staged 'The Responsive Eye' in 1965, a show of abstract paintings built on optical illusions?

The Museum of Modern Art put Bridget Riley's black-and-white 'Current' on the catalogue cover. A year earlier Time magazine had coined the nickname such eye-teasing painting still goes by.

Seen from its hollow side, a mask looks like a face whose eyes seem to do what as a visitor walks past?

They swivel to follow, turning even faster than the gaze of a flat portrait would. The mind so expects faces to bulge outward that it overrules shading and its own two-eyed depth sense.

Which Alfred Hitchcock film introduced the dolly zoom, where a background seems to stretch away behind a still subject?

Vertigo got the shot from cameraman Irmin Roberts, who moved the camera one way while zooming the lens the other. He went uncredited, and Jaws later reused the trick on a beach.

Salvador Dalí's painting of his wife Gala facing the sea becomes, from 20 metres away, a portrait of which president?

Abraham Lincoln emerges because Dalí built the canvas on a blocky grey mosaic of the president made by a Bell Labs researcher. His 1977 print of it became one of his most forged works.

Which animals name Akiyoshi Kitaoka's 2003 image of still coloured coils that appear to rotate slowly?

Snakes is the name because the bands of colour resemble coiled serpents. Nothing in the picture moves, and the drift is strongest at the edge of vision.

A secret portrait of Bonnie Prince Charlie looks like a meaningless smear until which object is stood on the painting?

A cylinder mirror gathers the smear into a face on its curved surface. The secret portrait is kept at the West Highland Museum, and a cone-shaped mirror can unlock other pictures of the kind.

Which British artist is known for chalk pavement drawings that look three-dimensional from one exact viewpoint?

Julian Beever stretches each drawing along the pavement so that it gains depth only from the right spot. He records the result in photographs, often with a passer-by posed inside the scene.

Which country's 1982 stamps honoured Oscar Reutersvärd, who as a schoolboy drew a triangle that could never be built?

Sweden honoured him almost 50 years after his 1934 doodle, a triangle built from cubes. He went on to draw more than 2,500 such figures.

Which philosopher made the duck-rabbit drawing famous by using it to contrast 'seeing that' with 'seeing as'?

Wittgenstein put the drawing in his Philosophical Investigations to show that a picture can stay exactly the same while what the viewer sees in it switches from one animal to the other.

Which term names edges that viewers see where nothing is drawn, as in Gaetano Kanizsa's bright phantom triangle?

Illusory contours make the phantom shape seem brighter than the page and nearer than the shapes around it, although the paper there is completely blank.

The Ames window, a trapezoid painted to resemble a rectangular frame, turns full circle yet appears to do what?

It seems to swing to and fro because viewers take the trapezoid for an ordinary rectangular window seen at an angle. Adelbert Ames Jr., famous for his distorted room, discovered the effect in 1947.

The Droste effect, a picture containing a smaller copy of itself again and again, is named after a Dutch brand of what?

Cocoa tins from Droste showed a nurse carrying a tray that held the very same tin, nurse and all. The 1904 design repeats for as far as the printing can resolve.