Optical Illusions
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Discover how much you know about optical illusions with our comprehensive free 44-question trivia quiz with answers. From classic visual tricks to modern perception puzzles, this quiz explores the fascinating world of how our eyes and brain can be deceived. Challenge yourself and learn surprising facts about optical illusions while testing your visual perception skills.
30 of 44 questions with answers and explanations. Play the quiz
Q 01Whose classification of visual illusions is widely used as an orientation?
Richard Gregory
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.
Q 02Which class of illusion is thought to result from over-stimulation of the eyes or brain, as with afterimages?
Physiological
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.
Q 03Optical illusions are commonly sorted into how many main classes?
Three
Illusions are commonly sorted into three main classes, and each class is further split into four kinds: ambiguities, distortions, paradoxes and fictions.
Q 04A stick appearing bent in water is the classic example of which class of illusion?
Physical
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.
Q 05Which class of illusion results from unconscious inferences and includes the most widely known examples?
Cognitive
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.
Q 06In the Müller-Lyer illusion, the orientation of what at the shafts' ends makes equal lines look different?
Fins
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.
Q 07In which year did Segall, Campbell and Herskovits publish their landmark cross-cultural Müller-Lyer study?
1963
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.
Q 08In what year did Franz Carl Müller-Lyer describe the line illusion named after him?
1889
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.
Q 09One theory explains the Müller-Lyer illusion as the brain misapplying which perceptual constancy?
Size
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.
Q 10In the Müller-Lyer figure, how does the shaft between two arrow 'tails' look compared with the one between 'heads'?
Longer
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.
Q 11Hermann Ebbinghaus, namesake of the circles illusion, pioneered the experimental study of what?
Memory
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.
Q 12The Ebbinghaus illusion is also known by what name, after the psychologist who popularized it in 1901?
Titchener Circles
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.
Q 13In the Ebbinghaus illusion, a central circle appears smaller when surrounded by what kind of circles?
Larger
Q 21Louis Albert Necker of the wire-frame cube was a cousin once removed of which linguist?
Ferdinand de Saussure
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.
Q 22An Ames room, a distorted room viewed through a peephole, makes people appear to do what as they walk between its corners?
Grow or shrink
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.
Q 23The Ames room illusion is viewed with one eye through a peephole, to remove depth cues from what?
Stereopsis
Distance matters too: the closer the surrounding circles sit to the centre, the bigger the centre looks.
Q 14The Ebbinghaus illusion has been found to deceive which group less often than university students?
Children aged 10 and under
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.
Q 15The Penrose triangle was first drawn in 1934, two decades before the Penroses, by which Swedish artist?
Oscar Reutersvärd
Oscar Reutersvärd; in his version the sides of the triangle are broken into cubes, and the Penroses devised theirs independently two decades later.
Q 16Which Dutch artist's 1961 lithograph 'Waterfall' features two elongated Penrose triangles?
M.C. Escher
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.
Q 17To which publication did Lionel and Roger Penrose submit their 1956 article on the Penrose triangle?
British Journal of Psychology
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.
Q 18What was the profession of Louis Albert Necker, who first published his wire-frame cube in 1832?
Crystallographer
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.
Q 19Staring 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
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.
Q 20To which Scottish physicist did Necker describe his reversing cube in an 1832 letter?
David Brewster
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.
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.
Q 24The true shape of an Ames room is typically that of an irregular what?
Hexahedron
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.
Q 25Which film trilogy used Ames-room-style forced-perspective sets to shrink some characters?
The Lord of the Rings
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.
Q 26A Fata Morgana, with its rapidly changing stack of images, is an elaborate form of which kind of mirage?
Superior
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.
Q 27Mirages are produced by which optical phenomenon?
Refraction
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.
Q 28Fata Morgana mirages took their name from sightings over which strait?
Strait of Messina
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.
Q 29'Mirage' comes via French from the Latin 'mirari', meaning what?
To look at, to wonder at
The word 'mirage' originates from the French 'mirer', which comes from the Latin 'mirari', meaning 'to look at, to wonder at.'
Q 30Which vision scientist invented random-dot stereograms in 1959 while working at Bell Labs on spotting camouflaged objects in spy-plane photos?
Béla Julesz
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.