Optical illusion

An optical illusion (also called a visual illusion[2]) is an illusion caused by the visual system and characterized by a visual percept that (loosely said) appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear[3] but a classification[1][4] proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect (where despite movement position remains unchanged). An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.

Pathological visual illusions arise from pathological changes in the physiological visual perception mechanisms causing the aforementioned types of illusions; they are discussed e.g. under visual hallucinations.

Checker shadow illusion
The checker shadow illusion. Although square A appears a darker shade of grey than square B, in the image the two have exactly the same luminance.
Grey square optical illusion proof2
Drawing a connecting bar between the two squares breaks the illusion and shows that they are the same shade.
Gregory categorization of illusions 1991
Gregory’s categorization of illusions [1]
Mach bands - animation
In this animation, Mach bands exaggerate the contrast between edges of the slightly differing shades of gray, as soon as they come in contact with one-another.

Physical visual illusions

A familiar phenomenon an example for a physical visual illusion are when mountains appear to be much nearer in clear weather with low humidity (Foehn) than they are. This is because haze is a cue for depth perception for far-away objects (Aerial perspective).

The classical example of a physical illusion is when a stick that is half immersed in water appears bent. This phenomenon has already been discussed by Ptolemy (ca. 150) [5] and was often a prototypical example for an illusion.

Physiological visual illusions

Physiological illusions, such as the afterimages[6] following bright lights, or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation or interaction with contextual or competing stimuli of a specific type—brightness, color, position, tile, size, movement, etc. The theory is that a stimulus follows its individual dedicated neural path in the early stages of visual processing and that intense or repetitive activity in that or interaction with active adjoining channels causes a physiological imbalance that alters perception.

The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach. Lateral inhibition, where in the receptive field of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands. Once a receptor is active, it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion the gray spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround.[7] Lateral inhibition has also been used to explain the Hermann grid illusion, but this has been disproved. [8] [9] [10] [11] [12] More recent empirical approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled.[13]

Cognitive illusions

"The Organ Player" – Pareidolia phenomenon in Neptune's Grotto stalactite cave (Alghero, Sardinia)

Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to "unconscious inferences", an idea first suggested in the 19th century by the German physicist and physician Hermann Helmholtz.[14] Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.

  1. Ambiguous illusions are pictures or objects that elicit a perceptual "switch" between the alternative interpretations. The Necker cube is a well-known example; another instance is the Rubin vase.
  2. Distorting or geometrical-optical illusions are characterized by distortions of size, length, position or curvature. A striking example is the Café wall illusion. Other examples are the famous Müller-Lyer illusion and Ponzo illusion.
  3. Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircase seen, for example, in M. C. Escher's Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join.
  4. Fictions are when a figure is perceived even though it is not in the stimulus.

Explanation of cognitive illusions

Perceptual organization

Two silhouette profile or a white vase
Reversible figures and vase, or the figure-ground illusion

To make sense of the world it is necessary to organize incoming sensations into information which is meaningful. Gestalt psychologists believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole.[15] Gestalt organization can be used to explain many illusions including the rabbit–duck illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in the figure–ground illusion the figure and ground are reversible.

In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa's Triangle. A floating white triangle, which does not exist, is seen. The brain has a need to see familiar simple objects and has a tendency to create a "whole" image from individual elements.[15] Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa's Triangle is based in evolutionary psychology and the fact that in order to survive it was important to see form and edges. The use of perceptual organization to create meaning out of stimuli is the principle behind other well-known illusions including impossible objects. Our brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which isn't there to that which is believable.

The Gestalt principles of perception govern the way we group different objects. Good form is where the perceptual system tries to fill in the blanks in order to see simple objects rather than complex objects. Continuity is where the perceptual system tries to disambiguate which segments fit together into continuous lines. Proximity is where objects that are close together are associated. Similarity is where objects that are similar are seen as associated. Some of these elements have been successfully incorporated into quantitative models involving optimal estimation or Bayesian inference. [16][17]

The double-anchoring theory, a popular but recent theory of lightness illusions, states that any region belongs to one or more frameworks, created by Gestalt grouping principles, and within each frame is independently anchored to both the highest luminance and the surround luminance. A spot's lightness is determined by the average of the values computed in each framework.[18]

Depth and motion perception

Vertical–horizontal illusion
The vertical–horizontal illusion where the vertical line is thought to be longer than the horizontal

Illusions can be based on an individual's ability to see in three dimensions even though the image hitting the retina is only two dimensional. The Ponzo illusion is an example of an illusion which uses monocular cues of depth perception to fool the eye. But even with two-dimensional images, the brain exaggerates vertical distances when compared with horizontal distances, as in the vertical-horizontal illusion where the two lines are exactly the same length.

In the Ponzo illusion the converging parallel lines tell the brain that the image higher in the visual field is farther away, therefore, the brain perceives the image to be larger, although the two images hitting the retina are the same size. The optical illusion seen in a diorama/false perspective also exploits assumptions based on monocular cues of depth perception. The M.C. Escher painting Waterfall exploits rules of depth and proximity and our understanding of the physical world to create an illusion. Like depth perception, motion perception is responsible for a number of sensory illusions. Film animation is based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. Likewise, when we are moving, as we would be while riding in a vehicle, stable surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly than smaller objects, like a car, although the larger object is actually moving faster. The phi phenomenon is yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession.

The ambiguity of direction of motion due to lack of visual references for depth is shown in the spinning dancer illusion. The spinning dancer appears to be moving clockwise or counterclockwise depending on spontaneous activity in the brain where perception is subjective. Recent studies show on the fMRI that there are spontaneous fluctuations in cortical activity while watching this illusion, particularly the parietal lobe because it is involved in perceiving movement.[19]

Color and brightness constancies

Simultaneous Contrast Illusion. The background is a color gradient and progresses from dark grey to light grey. The horizontal bar appears to progress from light grey to dark grey, but is in fact just one colour.

Perceptual constancies are sources of illusions. Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed. The luminosity of the object will appear brighter against a black field (that reflects less light) compared to a white field, even though the object itself did not change in luminosity. Similarly, the eye will compensate for color contrast depending on the color cast of the surrounding area.

In addition to the Gestalt principles of perception, water-color illusions contribute to the formation of optical illusions. Water-color illusions consist of object-hole effects and coloration. Object-hole effects occur when boundaries are prominent where there is a figure and background with a hole that is 3D volumetric in appearance. Coloration consists of an assimilation of color radiating from a thin-colored edge lining a darker chromatic contour. The water-color illusion describes how the human mind perceives the wholeness of an object such as top-down processing. Thus, contextual factors play into perceiving the brightness of an object.[20]


Shepard tables
"Shepard's tables" deconstructed. The two tabletops appear to be different, but they are the same size and shape.

Just as it perceives color and brightness constancies, the brain has the ability to understand familiar objects as having a consistent shape or size. For example, a door is perceived as a rectangle regardless of how the image may change on the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The "Shepard's table" illusion[21] is an example of an illusion based on distortions in shape constancy.

Future perception

Dipinto Dionysus, grattage su tela
Optical illusion

Researcher Mark Changizi of Rensselaer Polytechnic Institute in New York has a more imaginative take on optical illusions, saying that they are due to a neural lag which most humans experience while awake. When light hits the retina, about one-tenth of a second goes by before the brain translates the signal into a visual perception of the world. Scientists have known of the lag, yet they have debated how humans compensate, with some proposing that our motor system somehow modifies our movements to offset the delay.[22]

Changizi asserts that the human visual system has evolved to compensate for neural delays by generating images of what will occur one-tenth of a second into the future. This foresight enables humans to react to events in the present, enabling humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd.[23] In an interview with ABC Changizi said, "Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality."[24] For example, an illusion called the Hering illusion looks like bicycle spokes around a central point, with vertical lines on either side of this central, so-called vanishing point.[25] The illusion tricks us into thinking we are looking at a perspective picture, and thus according to Changizi, switches on our future-seeing abilities. Since we aren't actually moving and the figure is static, we misperceive the straight lines as curved ones. Changizi said:

Evolution has seen to it that geometric drawings like this elicit in us premonitions of the near future. The converging lines toward a vanishing point (the spokes) are cues that trick our brains into thinking we are moving forward—as we would in the real world, where the door frame (a pair of vertical lines) seems to bow out as we move through it—and we try to perceive what that world will look like in the next instant.[23]

Pathological visual illusions

A pathological visual illusion is a distortion of a real external stimulus[26] and are often diffuse and persistent. Pathological visual illusions usually occur throughout the visual field, suggesting global excitability or sensitivity alterations.[27] Alternatively visual hallucination is the perception of an external visual stimulus where none exists.[26] Visual hallucinations are often from focal dysfunction and are usually transient.

Types of visual illusions include oscillopsia, halos around objects, illusory palinopsia (visual trailing, light streaking, prolonged indistinct afterimages), akinetopsia, visual snow, micropsia, macropsia, teleopsia, pelopsia, Alice in Wonderland syndrome, metamorphopsia, dyschromatopsia, intense glare, blue field entoptic phenomenon, and purkinje trees.

These symptoms may indicate an underlying disease state and necessitate seeing a medical practitioner. Etiologies associated with pathological visual illusions include multiple types of ocular disease, migraines, hallucinogen persisting perception disorder, head trauma, and prescription drugs. If a medical work-up does not reveal a cause of the pathological visual illusions, the idiopathic visual disturbances could be analogous to the altered excitability state seen in visual aura with no migraine headache. If the visual illusions are diffuse and persistent, they often affect the patient's quality of life. These symptoms are often refractory to treatment and may be caused by any of the aforementioned etiologes, but are often idiopathic. There is no standard treatment for these visual disturbances.

List of illusions

There are a variety of different types of optical illusions. Many are included in the following list.

In art

Artists who have worked with optical illusions include M. C. Escher, Bridget Riley, Salvador Dalí, Giuseppe Arcimboldo, Patrick Bokanowski, Marcel Duchamp, Jasper Johns, Oscar Reutersvärd, Victor Vasarely and Charles Allan Gilbert. Contemporary artists who have experimented with illusions include Jonty Hurwitz, Sandro del Prete, Octavio Ocampo, Dick Termes, Shigeo Fukuda, Patrick Hughes, István Orosz, Rob Gonsalves, Gianni A. Sarcone, Ben Heine and Akiyoshi Kitaoka. Optical illusion is also used in film by the technique of forced perspective.

Op art is a style of art that uses optical illusions to create an impression of movement, or hidden images and patterns. Trompe-l'œil uses realistic imagery to create the optical illusion that depicted objects exist in three dimensions.

Cognitive processes hypothesis

The hypothesis claims that visual illusions occur because the neural circuitry in our visual system evolves, by neural learning, to a system that makes very efficient interpretations of usual 3D scenes based in the emergence of simplified models in our brain that speed up the interpretation process but give rise to optical illusions in unusual situations. In this sense, the cognitive processes hypothesis can be considered a framework for an understanding of optical illusions as the signature of the empirical statistical way vision has evolved to solve the inverse problem.[28]

Research indicates that 3D vision capabilities emerge and are learned jointly with the planning of movements. After a long process of learning, an internal representation of the world emerges that is well-adjusted to the perceived data coming from closer objects. The representation of distant objects near the horizon is less "adequate". In fact, it is not only the Moon that seems larger when we perceive it near the horizon. In a photo of a distant scene, all distant objects are perceived as smaller than when we observe them directly using our vision.


Motion aftereffect: this video produces a distortion illusion when the viewer looks away after watching it.


Ebbinghaus illusion: the orange circle on the left appears smaller than that on the right, but they are in fact the same size.

Café wall

Café wall illusion: the parallel horizontal lines in this image appear sloped.


Checker version: the diagonal checker squares at the larger grid points make the grid appear distorted.


Lilac chaser: if the viewer focuses on the black cross in the center, the location of the disappearing dot appears green.

Motion illusion in star arrangement

Motion illusion: contrasting colors create the illusion of motion.

Subjectively constructed water-color

Watercolor illusion: this shape's yellow and blue border create the illusion of the object being pale yellow rather than white[29]

Optical illusion - subjectively constructed cyan sqare filter above blue cirles

Subjective cyan filter, left: subjectively constructed cyan square filter above blue circles, right: small cyan circles inhibit filter construction[30][31]

Pinna's illusory intertwining effect

Pinna's illusory intertwining effect[32] and Pinna illusion (scholarpedia).[33](The picture shows squares spiralling in, although they are arranged in concentric circles.)

Politeness simulation (LOC cph.3g08085)

Optical illusion disc which is spun displaying the illusion of motion of a man bowing and a woman curtsying to each other in a circle at the outer edge of the disc, 1833

Hybrid image decomposition

A hybrid image constructed from low-frequency components of a photograph of Marilyn Monroe (left inset) and high-frequency components of a photograph of Albert Einstein (right inset). The Einstein image is clearer in the full image.

Roman geometric mosaic

An ancient Roman geometric mosaic. The cubic texture induces a Necker-cube-like optical illusion.

a set of colorful spinning disks that create illusion. The disks appear to move backwards and forwards in different regions.

Revolving circles

Pinna-Brelstaff illusion: the two circles seem to move when the viewer's head is moving forwards and backwards while looking at the black dot.[34]

Spinning Dancer

The Spinning Dancer appears to move both clockwise and counter-clockwise

See also

  • Portal-puzzle.svg Optical illusions portal


  1. ^ a b Gregory, Richard (1991). "Putting illusions in their place". Perception. 20 (1): 1–4. doi:10.1068/p200001. PMID 1945728.
  2. ^ In the scientific literature the term "visual illusion" is preferred because the older term gives rise to the assumption that the optics of the eye were the general cause for illusions (which is only the case for so-called physical illusions). "Optical" in the term derives from the Greek optein = "seeing", so the term refers to an "illusion of seeing", not to optics as a branch of modern physics. A regular scientific source for illusions is the journals Perception and i-Perception
  3. ^ Bach, Michael; Poloschek, C. M. (2006). "Optical Illusions" (PDF). Adv. Clin. Neurosci. Rehabil. 6 (2): 20–21.
  4. ^ Gregory, Richard L. (1997). "Visual illusions classified" (PDF). Trends in Cognitive Sciences. 1 (5): 190–194. doi:10.1016/s1364-6613(97)01060-7. PMID 21223901.
  5. ^ Wade, Nicholas J. (1998). A natural history of vision. Cambridge, MA: MIT Press.
  6. ^ "After Images". worqx.com. Archived from the original on 2015-04-22.
  7. ^ Pinel, J. (2005) Biopsychology (6th ed.). Boston: Allyn & Bacon. ISBN 0-205-42651-4
  8. ^ Lingelbach B, Block B, Hatzky B, Reisinger E (1985). "The Hermann grid illusion -- retinal or cortical?". Perception. 14 (1): A7.
  9. ^ Geier J, Bernáth L (2004). "Stopping the Hermann grid illusion by simple sine distortion". Perception. Malden Ma: Blackwell. pp. 33–53. ISBN 978-0631224211.
  10. ^ Schiller, Peter H.; Carvey, Christina E. (2005). "The Hermann grid illusion revisited". Perception. 34 (11): 1375–1397. doi:10.1068/p5447. PMID 16355743. Archived from the original on 2011-12-12. Retrieved 2011-10-03.
  11. ^ Geier J, Bernáth L, Hudák M, Séra L (2008). "Straightness as the main factor of the Hermann grid illusion". Perception. 37 (5): 651–665. doi:10.1068/p5622. PMID 18605141.
  12. ^ Bach, Michael (2008). "Die Hermann-Gitter-Täuschung: Lehrbucherklärung widerlegt (The Hermann grid illusion: the classic textbook interpretation is obsolete)". Ophthalmologe. 106 (10): 913–917. doi:10.1007/s00347-008-1845-5. PMID 18830602.
  13. ^ Howe, Catherine Q.; Yang, Zhiyong; Purves, Dale (2005). "The Poggendorff illusion explained by natural scene geometry". PNAS. 102 (21): 7707–7712. doi:10.1073/pnas.0502893102. PMC 1093311. PMID 15888555.
  14. ^ David Eagleman (April 2012). Incogito: The Secret Lives of the Brain. Vintage Books. pp. 33–. ISBN 978-0-307-38992-3. Archived from the original on 12 October 2013. Retrieved 14 August 2013.
  15. ^ a b Myers, D. (2003). Psychology in Modules, (7th ed.) New York: Worth. ISBN 0-7167-5850-4
  16. ^ Yoon Mo Jung and Jackie (Jianhong) Shen (2008), J. Visual Comm. Image Representation, 19(1):42-55, First-order modeling and stability analysis of illusory contours.
  17. ^ Yoon Mo Jung and Jackie (Jianhong) Shen (2014), arXiv:1406.1265, Illusory shapes via phase transition Archived 2017-11-24 at the Wayback Machine.
  18. ^ Bressan, P (2006). "The Place of White in a World of Grays: A Double-Anchoring Theory of Lightness Perception". Psychological Review. 113 (3): 526–553. doi:10.1037/0033-295x.113.3.526. PMID 16802880.
  19. ^ Bernal, B., Guillen, M., & Marquez, J. (2014). The spinning dancer illusion and spontaneous brain fluctuations: An fMRI study. Neurocase (Psychology Press), 20(6), 627-639.
  20. ^ Tanca, M.; Grossberg, S.; Pinna, B. (2010). "Probing Perceptual Antinomies with the Watercolor Illusion and Explaining How the Brain Resolves Them" (PDF). Seeing & Perceiving. 23 (4): 295–333. CiteSeerX doi:10.1163/187847510x532685. PMID 21466146. Archived (PDF) from the original on 2017-09-21.
  21. ^ Bach, Michael (4 January 2010) [16 August 2004]. "Shepard's "Turning the Tables"". michaelbach.de. Michael Bach. Archived from the original on 27 January 2010. Retrieved 27 January 2010.
  22. ^ Bryner, Jeanna. "Scientist: Humans Can See Into Future". foxnews.com. Retrieved 13 July 2018.
  23. ^ a b Key to All-Optical Illusions Discovered Archived 2008-09-05 at the Wayback Machine, Jeanna Bryner, Senior Writer, LiveScience.com 6/2/08. His research on this topic is detailed in the May/June 2008 issue of the journal Cognitive Science.
  24. ^ NIERENBERG, CARI (2008-02-07). "Optical Illusions: When Your Brain Can't Believe Your Eyes". ABC News. Retrieved 13 July 2018.
  25. ^ Barile, Margherita. "Hering Illusion". mathworld. Wolfram. Retrieved 13 July 2018.
  26. ^ a b Pelak, Victoria. "Approach to the patient with visual hallucinations". www.uptodate.com. Archived from the original on 2014-08-26. Retrieved 2014-08-25.
  27. ^ Gersztenkorn, D; Lee, AG (Jul 2, 2014). "Palinopsia revamped: A systematic review of the literature". Survey of Ophthalmology. 60 (1): 1–35. doi:10.1016/j.survophthal.2014.06.003. PMID 25113609.
  28. ^ Gregory, Richard L. "Knowledge in perception and illusion" (PDF). Archived (PDF) from the original on 2005-04-04.
  29. ^ Bangio Pinna; Gavin Brelstaff; Lothar Spillman (2001). "Surface color from boundaries: a new watercolor illusion". Vision Research. 41 (20): 2669–2676. doi:10.1016/s0042-6989(01)00105-5. PMID 11520512.
  30. ^ Hoffmann, Donald D. (1998). Visual Intelligence. How we create what we see. Norton., p.174
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  32. ^ Pinna, B., Gregory, R.L. (2002). "Shifts of Edges and Deformations of Patterns". Perception. 31 (12): 1503–1508. doi:10.1068/p3112pp. PMID 12916675.CS1 maint: Multiple names: authors list (link)
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Further reading

External links


An antitail is a spike projecting from a comet's coma which seems to go towards the Sun, and thus geometrically opposite to the other tails: the ion tail and the dust tail. However, this phenomenon is an optical illusion that is seen from the Earth. The antitail is formed of larger dust particles, which are less affected by the Sun's radiation pressure and tend to remain roughly in the comet's orbital plane and eventually form a disc along the comet's orbit due to the ejection speed of the particles from the comet's surface. As Earth passes through the comet's orbital plane, this disc is seen side on, and appears as the characteristic spike. The other side of the disc can sometimes be seen, though it tends to be lost in the dust tail. The antitail is therefore normally visible for a brief interval only when Earth passes through the comet's orbital plane.Most comets do not develop sufficiently for an antitail to become visible, but notable comets that did display antitails include Comet Arend–Roland in 1957, Comet Hale–Bopp in 1997, and Comet PANSTARRS in 2013.

Auditory illusion

An auditory illusion is an illusion of hearing, the aural equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or "impossible" sounds. In short, auditory illusions highlight areas where the human ear and brain, as organic survival tools, differ from perfect audio receptors (for better or for worse).

Bezold effect

The Bezold effect is an optical illusion, named after a German professor of meteorology, Wilhelm von Bezold (1837–1907), who discovered that a color may appear different depending on its relation to adjacent colors.

It happens when small areas of color are interspersed. An assimilation effect called the von Bezold spreading effect, similar to spatial color mixing, is achieved.

The opposite effect is observed when large areas of color are placed adjacent to each other, resulting in color contrast.

Café wall illusion

The café wall illusion is a geometrical-optical illusion in which the parallel straight dividing lines between staggered rows with alternating black and white "bricks" appear to be sloped.

It was first described under the name Kindergarten illusion in 1898, and re-discovered in 1973 by Richard Gregory. According to Gregory, this effect was observed by a member of his laboratory, Steve Simpson, in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol. It is a variant of the shifted-chessboard illusion originated by Hugo Münsterberg.In the construction of the optical illusion often each "brick" is surrounded by a layer of "mortar" intermediate between the dark and light colours of the "bricks".In the first attempt at its deconstruction, the illusion was ascribed largely to irradiation, the light spread from dark to bright zones in the retinal image, and the image disappears when black and white are replaced by different colours of the same brightness. But a component of the illusion remains even when all optical and retinal components are factored out. Contrast polarities seem to be the determining factor in the tilt's direction.

Checker shadow illusion

The checker shadow illusion is an optical illusion published by Edward H. Adelson, Professor of Vision Science at MIT in 1995.

Delboeuf illusion

The Delboeuf illusion is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size have been placed near to each other and one is surrounded by an annulus; the surrounded circle then appears larger than the non-surrounded circle if the annulus is close, while appearing smaller than the non-surrounded circle if the annulus is distant. A 2005 study suggests it is caused by the same visual processes that cause the Ebbinghaus illusion.

Fraser spiral illusion

The Fraser spiral illusion is an optical illusion that was first described by the British psychologist Sir James Fraser (1863 – 1936) in 1908.The illusion is also known as the false spiral, or by its original name, the twisted cord illusion. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.

The visual distortion is produced by combining a regular line pattern (the circles) with misaligned parts (the differently colored strands). Zöllner's illusion and the café wall illusion are based on a similar principle, like many other visual effects, in which a sequence of tilted elements causes the eye to perceive phantom twists and deviations.

The illusion is augmented by the spiral components in the checkered background. It is a unique illusion, where the observer can verify the concentric strands manually. When the strands are highlighted in a different colour, it becomes obvious to the observer that no spiral is present.

Geometrical-optical illusions

Geometrical-optical illusions are visual illusions, also optical illusions, in which the geometrical properties of what is seen differ from those of the corresponding objects in the visual field.

Gravity hill

A gravity hill, also known as a magnetic hill, mystery hill, mystery spot, gravity road, or anti-gravity hill, is a place where the layout of the surrounding land produces an optical illusion, making a slight downhill slope appear to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill against gravity. There are hundreds of recognised gravity hills around the world.

The slope of gravity hills is an optical illusion, although sites are often accompanied by claims that magnetic or supernatural forces are at work. The most important factor contributing to the illusion is a completely or mostly obstructed horizon. Without a horizon, it becomes difficult to judge the slope of a surface, as a reliable reference is missing. Objects one would normally assume to be more or less perpendicular to the ground (such as trees) may actually be leaning, offsetting the visual reference.The illusion is similar to the Ames room, in which objects can also appear to roll against gravity.

A similar phenomenon—an uphill road that appears flat—is known in bicycle racing as a "false flat".

Impossible object

An impossible object (also known as an impossible figure or an undecidable figure) is a type of optical illusion. It consists of a two-dimensional figure which is instantly and subconsciously interpreted by the visual system as representing a projection of a three-dimensional object.

In most cases the impossibility becomes apparent after viewing the figure for a few seconds. However, the initial impression of a 3D object remains even after it has been contradicted. There are also more subtle examples of impossible objects where the impossibility does not become apparent spontaneously and it is necessary to consciously examine the geometry of the implied object to determine that it is impossible.

The unsettling nature of impossible objects occurs because of our natural desire to interpret 2D drawings as three-dimensional objects. This is why a drawing of a Necker cube would be most likely seen as a cube, rather than "two squares connected with diagonal lines, a square surrounded by irregular planar figures, or any other planar figure." With an impossible object, looking at different parts of the object makes one reassess the 3D nature of the object, which confuses the mind.Impossible objects are of interest to psychologists, mathematicians and artists without falling entirely into any one discipline.

Jastrow illusion

The Jastrow illusion is an optical illusion attributed to the Polish-American psychologist Joseph Jastrow. This optical illusion is known under different names: Ring-Segment illusion, Jastrow illusion, Wundt area illusion or Wundt-Jastrow illusion.The illusion also occurs in the real world. In the illustration on the right, the two toy railway tracks are identical, although the lower one appears to be larger. There are three competing theories on how this illusion occurs.This illusion is often included in magic kits and several versions are sold in magic shops and is commonly known under the name Boomerang Illusion.

List of optical illusions

This is a list of optical illusions.

Mean line

In typography, the mean line, also called the midline, is half the distance from the baseline to the cap height. This may or may not be the x-height, depending on the design of the lower case letters. A very high or very low x-height may mean that the midline is above or below the x-height.

This confusion has been perpetuated by books, and websites, on typography which copied from books on type back to the original book or source on type that first failed to clarify this difference.

Round glyphs will break (overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing; a rounded shape will appear visually smaller than flat-topped (or bottomed) shapes of equal height, due to an optical illusion.

Orbison illusion

The Orbison illusion (or Orbison's illusion) is an optical illusion first described by American psychologist William Orbison (1912–1952) in 1939.

The illusion consists of a two dimensional figure, such as a circle or square, superimposed over a background of radial lines or concentric circles. The result is an optical illusion in which both the figure and the rectangle which contains it appear to distorted; in particular, squares appear slightly bulged, circles appear elliptical, and the containing rectangle appears tilted.

Sander illusion

The Sander illusion or Sander's parallelogram is an optical illusion described by the German psychologist Friedrich Sander (1889–1971) in 1926. However, it had been published earlier by Matthew Luckiesh in his 1922 book Visual Illusions: Their Causes, Characteristics, and Applications.The diagonal line bisecting the larger, left-hand parallelogram appears to be considerably longer than the diagonal line bisecting the smaller, right-hand parallelogram, but is the same length.

One possible reason for this illusion is that the diagonal lines around the blue lines give a perception of depth, and when the blue lines are included in that depth, they are perceived as different lengths.


Spectropia (full title Spectropia, or, surprising spectral illusions showing ghosts everywhere and of any colour) is an optical illusion book by J. H. Brown, first published in 1864.

The dress

The dress is a photograph that became a viral internet sensation on 26 February 2015, when viewers disagreed over whether the dress pictured was coloured black and blue, or white and gold. The phenomenon revealed differences in human colour perception, which have been the subject of ongoing scientific investigations into neuroscience and vision science, with a number of papers published in peer-reviewed science journals.

The photo originated from a washed-out colour photograph of a dress posted on the social networking service Tumblr. Within the first week after the surfacing of the image, more than 10 million tweets mentioned the dress, using hashtags such as #thedress, #whiteandgold, and #blackandblue. Although the actual colour was eventually confirmed as black and blue, the image prompted many discussions, with users debating their opinions on the colour and how they perceived the dress in the photograph as a certain colour. Members of the scientific community began to investigate the photo for fresh insights into human color vision.

The dress itself, which was identified as a product of the retailer Roman Originals, experienced a major surge in sales as a result of the incident. The retailer also produced a one-off version of the dress in white and gold as a charity campaign.

Time Requiem

Time Requiem is a Swedish neoclassical power metal band created by Richard Andersson in 2001. Andersson created the band that summer, when he felt that his previous project - Majestic - had come to a dead end.

Wundt illusion

The Wundt illusion is an optical illusion that was first described by the German psychologist Wilhelm Wundt in the 19th century. The two red vertical lines are both straight, but they may look as if they are bowed inwards to some observers. The distortion is induced by the crooked lines on the background, as in the Orbison illusion. The Hering illusion produces a similar, but inverted effect.

Optical illusions (list)
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