44 Fun Facts About Optical Illusions

Whose classification of visual illusions is widely used as an orientation?

Richard Gregory's classification of illusions, with each class split into ambiguities, distortions, paradoxes and fictions, is the standard orientation; Gregory also championed the view that perception works by testing hypotheses.

Which class of illusion is thought to result from over-stimulation of the eyes or brain, as with afterimages?

Physiological illusions such as afterimages come from excessive or repetitive stimulation of a dedicated neural channel early in visual processing, leaving an imbalance that alters what is seen.

Optical illusions are commonly sorted into how many main classes?

Illusions are commonly sorted into three main classes, and each class is further split into four kinds: ambiguities, distortions, paradoxes and fictions.

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

The bent stick is a physical illusion: it arises from the behaviour of light itself, refracting at the water surface before it reaches the eye, not from anything the eye or brain does.

Which class of illusion results from unconscious inferences and includes the most widely known examples?

Cognitive illusions arise from unconscious inferences about the world: the Ponzo and Müller-Lyer figures are typical distortions of this class, and the rabbit–duck drawing a typical ambiguity.

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.

In which year did Segall, Campbell and Herskovits publish their landmark cross-cultural Müller-Lyer study?

Segall, Campbell and Herskovits published the cross-cultural study in Science in 1963, proposing the 'carpentered world' idea that environments of straight lines and right angles shape susceptibility.

In what year did Franz Carl Müller-Lyer describe the line illusion named after him?

Franz Carl Müller-Lyer, a German sociologist and psychiatrist, published the illusion in 1889; the figure is a line whose ends are capped by inward or outward chevrons.

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 configuration really does tend to come from physically longer sources, which may be why the brain guesses as it does.

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

A copy of Fechner's Elements of Psychophysics found in a London second-hand bookshop spurred him on; he began his memory experiments in 1879.

The Ebbinghaus illusion is also known by what name, after the psychologist who popularized it in 1901?

The Ebbinghaus illusion was popularized in the English-speaking world by Edward B. Titchener in a 1901 textbook, leading to its alternative name, Titchener circles.

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

Distance matters too: the closer the surrounding circles sit to the centre, the bigger the centre looks.

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.

The Penrose triangle was first drawn in 1934, two decades before the Penroses, by which Swedish artist?

Oscar Reutersvärd; in his version the sides of the triangle are broken into cubes, and the Penroses devised theirs independently two decades later.

Which Dutch artist's 1961 lithograph 'Waterfall' features two elongated Penrose triangles?

M.C. Escher's 1961 lithograph Waterfall runs a zigzag watercourse along the long sides of two Penrose triangles so it ends higher than it began; Escher's work had partly inspired the Penroses.

To which publication did Lionel and Roger Penrose submit their 1956 article on the Penrose triangle?

Lionel and Roger Penrose sent their paper to the British Journal of Psychology in 1956; it appeared in 1958, illustrated with the triangle and the Penrose stairs.

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; focusing on the base tends to bring one interpretation, focusing on the left side the other, and blinking often triggers a switch.

To which Scottish physicist did Necker describe his reversing cube in an 1832 letter?

Necker described the reversal in an 1832 letter to Sir David Brewster, printed in the Philosophical Magazine, after engraved crystal drawings seemed to flip in depth as he studied them.

Louis Albert Necker of the wire-frame cube was a cousin once removed of which linguist?

Necker's full name was Louis Albert Necker de Saussure: his mother Albertine was a daughter of the Alpine scientist Horace-Bénédict de Saussure, from whom the linguist Ferdinand de Saussure also descended.

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; viewers know an adult cannot change like that, yet the effect persists as they walk between the true near and far corners.

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 Ames-room-style forced-perspective sets to shrink some characters?

Peter Jackson's The Lord of the Rings trilogy built forced-perspective sets in the Ames-room manner, such as Bag End, so hobbits appeared 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: a strong temperature inversion forms an atmospheric duct in which light curves more sharply than the Earth's surface, stacking distant objects above the horizon.

Mirages are produced by which optical phenomenon?

Refraction; a mirage is a displaced image of distant objects or the sky, formed as light rays bend through layers of air at different temperatures.

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 via French from the Latin 'mirari', meaning what?

The word 'mirage' originates from the French 'mirer', which comes from the Latin 'mirari', meaning 'to look at, to wonder at.'

Which vision scientist invented random-dot stereograms in 1959 while working at Bell Labs on spotting camouflaged objects in spy-plane photos?

Béla Julesz showed binocular depth works even without visible contours in either eye's image; his student Christopher Tyler made the first single-image autostereogram in 1979 on an Apple II.

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.

The popular Magic Eye books of the 1990s use what type of image?

Magic Eye books are random-dot autostereograms: programmer Tom Baccei and artist Cheri Smith created the first colour ones in 1991, and the 1993 book series became a bestseller.

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?

A version was described by Hugo Münsterberg in 1894; the current name dates from 1973, after a lab member spotted the effect on a café at the bottom of St Michael's Hill, Bristol.

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

Psychologist Joseph Jastrow used it, and Ludwig Wittgenstein made it famous in Philosophical Investigations to distinguish 'seeing that' from 'seeing as'.

Staring at a green patch, then a blank wall, produces a magenta what?

A negative afterimage appears because cone cells tuned to green adapt and fire less, so the complementary magenta shows when you look at a blank surface.

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.

Which effect makes two parallel lines look bowed outwards against a radial background of spokes?

Ewald Hering described it in 1861; the Wundt figure is its counterpart, in which the lines seem to bow inwards.

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

The autokinetic effect; Alexander von Humboldt recorded it in 1799 when a star seemed to wander in the night sky, though he believed the movement was real.

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 the brain fills in a bright triangle with illusory edges; Gaetano Kanizsa published it in 1955.

Which flower-like illusion with shaded petals makes its inner background look far brighter?

Asahi means 'morning sun' in Japanese, and the effect is strong enough that viewers' pupils actually constrict when they look at the centre.

Which journal editor received Zöllner's line illusion in 1860 and spotted a related illusion of his own?

Johann Poggendorff, editor of Annalen der Physik, received astrophysicist Zöllner's figure in 1860 and noticed the separate Poggendorff illusion, in which a diagonal interrupted by a bar looks misaligned.

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.