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The missing square puzzle is an optical illusion used in mathematics classes to help students reason about geometrical figures; or rather to teach them not to reason using figures, but to use only textual descriptions and the axioms of geometry. It depicts two arrangements made of similar shapes in slightly different configurations.
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.
The Moon illusion is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. Motion aftereffect: Motion illusion: Müller-Lyer illusion: The Müller-Lyer illusion is an optical illusion consisting of a stylized arrow. Multistable perception: Necker cube
The chessboard paradox [1] [2] or paradox of Loyd and Schlömilch [3] is a falsidical paradox based on an optical illusion. A chessboard or a square with a side length of 8 units is cut into four pieces. Those four pieces are used to form a rectangle with side lengths of 13 and 5 units.
The widely accepted interpretation of, e.g. the Poggendorff and Hering illusions as manifestation of expansion of acute angles at line intersections, is an example of successful implementation of a "bottom-up," physiological explanation of a geometrical–optical illusion. Ponzo illusion in a purely schematic form and, below, with perspective clues
Interactive SVG of The Disappearing Bicyclist – in the SVG file, move the pointer to rotate the disc. A vanishing puzzle is a mechanical optical illusion comprising multiple pieces which can be rearranged to show different versions of a picture depicting several objects, the number of which depending on the arrangement of the pieces.
An illusion closely related to the checker shadow illusion, which also relies on using implied visual shadows to seemingly darken a brighter region to the same color as a well-lit dark region, involves two squares placed at an angle, with the darker square being lit and the lighter square at an angle which receives poor light. [2]
Shepard first published this optical paradox in his 1990 book Mind Sights (page 79) giving it the name "L'egs-istential Quandary". [2] It is the first entry in his chapter on "Figure-ground impossibilities". The pen-and-ink drawing is based on a dream Shepard had in 1974, and on the pencil sketch he made when he woke up. [2]