Optical Illusions
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Discover how much you know about optical illusions with our comprehensive free 94-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 94 questions with answers and explanations. Play the quiz
Q 01A 2015 viral photo split viewers over whether a dress was white and gold or which other pair of colours?
Blue and black
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.
Q 02Which class of illusion is thought to come from overstimulating the eyes or brain, like the spots left by a camera flash?
Physiological
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.
Q 03Impossible objects like the Penrose triangle are which kind of cognitive illusion?
Paradoxes
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.
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 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.
Q 05Rubin's famous reversible figure can be seen as two faces in profile or as which object between them?
A vase
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.
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 07A 1963 study linked the Müller-Lyer illusion to growing up among what?
Right-angled buildings
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.
Q 08The Moon illusion makes the Moon look largest when it is in which position?
Near the horizon
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.
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 figure really does tend to come from physically longer edges in a scene, 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
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.
Q 12In the Ponzo illusion, two identical bars cross a pair of lines converging towards the top; what do you see?
The upper looks longer
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.
Q 13In the Ebbinghaus illusion, a central circle looks undersized when surrounded by what kind of circles?
Larger
Q 21Which object is hidden as a stretched smear across the floor of Holbein's 1533 painting 'The Ambassadors'?
A skull
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.
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: 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.
Q 23The Ames room illusion is viewed with one eye through a peephole, to remove depth cues from what?
Stereopsis
Larger circles make the centre look smaller, and distance matters too: the closer the surrounding ring sits, the bigger the central circle appears.
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 15Which 1960s art movement, led by Bridget Riley and Victor Vasarely, used geometric patterns that seem to vibrate and move?
Op art
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.
Q 16Which Dutch artist's 1961 lithograph 'Waterfall' sends water around a circuit built from Penrose triangles?
M.C. Escher
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.
Q 17What is the term for seeing faces in clouds, car fronts or the surface of the Moon?
Pareidolia
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'.
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: 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.
Q 20Why don't you normally notice the blind spot where the optic nerve leaves each retina?
The brain fills it in
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.
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 forced-perspective sets to make some characters look far shorter than others in the same shot?
The Lord of the Rings
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.
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: under a strong temperature inversion, distant ships or coasts appear to float, stretched and stacked in alternating upright and inverted images.
Q 27Mirages are produced by which optical phenomenon?
Refraction
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.
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 from the Latin 'mirari', also the root of 'miracle' and 'admire', meaning what?
To wonder at
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.
Q 30Which French term names paintings so lifelike that viewers mistake a painted door or curtain for the real thing?
Trompe-l'œil
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.