Hidden contours

An optical illusion is an image or scene that creates a difference between its physical features and the way it is perceived. Context, perspective, contrast and visual adaptation can all contribute to the effect.

Vision interprets light using surrounding shapes, colours, depth cues and prior experience. Illusions reveal situations in which these normally useful processes produce surprising interpretations. Different illusions involve different mechanisms.

Through Mirror includes size and geometry illusions, light and colour effects, ambiguous figures, impossible objects, motion illusions and hidden contours. The collection also explores illusion art, architecture, fractals, animal camouflage and other senses.

Open an entry for its viewing instructions, explanation, history and sources. Gallery images can be enlarged, reversible pictures can be rotated, and animations play when selected. Some demonstrations link to their original source.

An architectural illusion changes the apparent shape, depth or extent of a real place through painting, perspective, geometry or reflection. Its effect often depends on where the observer stands.

The cathedral’s famous apparent dome is a painted illusion by Antonino Emanuele, dating to 1739. The painting makes a flat ceiling appear to rise into a dome from the intended viewpoint.

Forced perspective changes scale and alignment to suggest a different depth or size. Borromini’s corridor at Palazzo Spada uses a compressed construction that appears longer from its intended viewing position.

Animal camouflage includes colour, pattern, shape and other features that make an animal harder to detect or recognise. Its effectiveness depends on the background, lighting and the observer’s vision.

Camouflage reduces detection or recognition. Resembling another object, such as a leaf, can help an animal be mistaken for part of its surroundings; resemblance can serve other biological functions too.

Cuttlefish can change visible skin patterns and texture. Their colour changes involve pigment-containing structures and light-reflecting components, helping them alter their appearance against different backgrounds.

A fractal is a mathematical structure with detail or related patterns across different scales. Some fractals repeat an exact construction; others show more varied forms of self-similarity.

Not necessarily. Through Mirror includes fractals because their repetition, intricate boundaries and nested patterns challenge visual intuition. Their geometry does not have to create a perceptual error.

The Mandelbrot set is defined by repeatedly applying a mathematical rule to complex numbers and checking whether the results remain bounded. Images show its intricate boundary and related patterns at different scales.

Trompe-l’œil means deceiving the eye. In painting, carefully arranged perspective, shading and familiar objects can make a flat surface appear to contain real depth or objects.

Anamorphosis is a distorted image that becomes recognisable from a particular viewpoint or with an appropriate reflective surface. Holbein’s The Ambassadors includes a skull revealed by an oblique view.

Arcimboldo arranged recognisable objects such as fruit, flowers and books into the features of a face. Some works also reveal a second interpretation when the painting is turned upside down.

Geometric illusions change the apparent size, length, angle or alignment of shapes. Surrounding lines and perspective cues can make equal lengths look unequal or parallel lines appear tilted.

Yes. The horizontal boundaries are parallel, but the alternating arrangement of light and dark tiles makes them appear to slope.

Follow the entry’s instructions, then compare the relevant lengths with a ruler or cover the surrounding context. This can separate the physical measurement from the visual impression.

The visual system judges a surface in relation to nearby contrast and apparent lighting. The same image value can look lighter or darker depending on its surroundings.

Yes. The labelled squares have the same grey value. The depicted shadow and checkerboard context make them appear different; isolating the squares reveals the match.

Mach bands are apparent light and dark strips near changes in brightness. They illustrate how the visual system responds to contrast at boundaries.

A colour afterimage is a lingering visual impression after looking away from a coloured stimulus. Adaptation in the visual system can make a different colour appear against a neutral background.

Colour perception depends partly on surrounding colours and contrast. An unchanged patch can look different when placed in a different visual context.

Use the viewing instructions on the entry page. Look at the specified point, then shift to the suggested neutral area. You can stop whenever you wish; the effect varies between viewers.

An ambiguous figure supports more than one interpretation of the same image. A viewer may switch between the interpretations even though the picture stays unchanged.

The same outline can be read as a duck facing one direction or a rabbit facing the other. The shape that looks like a beak can also look like ears.

The central region can be seen as a vase, while the surrounding regions can be seen as two facing profiles. The shared boundary changes meaning as foreground and background switch.

An impossible object is a drawing whose parts suggest familiar three-dimensional shapes but cannot all connect as they appear to. The contradiction becomes clearer when you trace the whole structure.

It cannot exist as the ordinary solid the drawing suggests. A specially arranged model can create the appearance from one viewpoint, but its actual structure differs from the pictured object.

The drawing joins plausible stair segments into an apparent loop that continually rises or falls. In ordinary space, a closed circuit cannot gain height at every step and return to its starting level.

Certain arrangements of contrast and pattern can produce apparent movement, especially away from the centre of attention. A motion illusion does not always require an animated image.

No. The silhouette has ambiguous depth cues and can be interpreted as turning in either direction. Its perceived rotation does not establish a personality type or a dominant brain hemisphere.

No. Through Mirror initially shows still previews. Open a demonstration and select Play animation when you want to see it move.

Illusory contours are apparent edges that are not explicitly drawn. Surrounding shapes can make a viewer perceive a continuous boundary or a surface.

The bright triangle appears to have clear edges, but those edges are not drawn. The surrounding cut-out shapes suggest a triangle covering parts of them.

Vision uses the relationships between visible edges and surfaces to interpret partially hidden objects. Illusory-contour demonstrations show how a coherent shape can be perceived from incomplete cues.

An auditory illusion is a surprising perception of sound influenced by context, arrangement or interpretation. Examples include ambiguous melodies and apparent changes in pitch.

Entries explain the effects and link to their original demonstrations when audio is hosted elsewhere. Follow the source link and the instructions for the particular effect.

A tactile illusion is a surprising experience of touch or bodily sensation. Adaptation, neighbouring stimulation, body position and expectations can influence what is felt.

Some demonstrations explore touch alone, while others involve interactions between touch and vision. The entry’s instructions explain which senses and conditions are involved.

A multisensory illusion involves interactions between senses. What a person sees, hears or feels can change how information from another sense is interpreted.

The McGurk effect occurs when seeing speech movements influences the speech sound a listener perceives. Combining mismatched visual and auditory speech can produce a different perceived syllable.