| An optical illusion is an error in visual sight: the improper estimation of stretch lengths, dimensions of angles and the color of a drawn object. The causes of those errors lie in the peculiarities of the physiology of sight, as well as the psychology of perception.
The intentional distortion of perspective in paintings, evoke special effects, better known in the works of Morris Asher.
Several optic illusions are studied within Gestalt-psychology - the perception of size in the illusion.
The illusion of movement
The illusion of color perception. One ring seems darker than the other, but in fact both are the same. Spoonerism –“duck-hare”
Physiological illusions
Illusion of grating
Morris Asher. Waterfall (1961), Example of impossible objects
Perception of size
If not to measure visual estimations with a ruler, it happens that one can make a mistake of 25% and more
Visual estimations of geometric real values, very much depend upon the nature of the background and the image. This is relevant to lengths, squares and the radii of curve. We can also show that the above said is true to angles, shapes etc.
Illusion of color perception
100 years before already it was known that when an image appears in the retina comprising of bright and dark areas, light from the brightly lighted areas seem to spill over on the dark areas. This phenomenon is called irradiation.
Illusion of movement
The quiescent image seems to be moving.
The most well known impossible figures and objects are in the works of Morris Asher. While studying such pictures, every separate feature is very much probable, yet when attempting to look for a line, it happens that this line, for instance, is not an external wall but an internal one. Other well known works belong to Penrose, Ruthersword and the models of Eims.
Spoonerism is a type of optic illusion, where the feature of a perceived object depends upon the direction of sight. One among those illusions is the “duck hare”: the image can be described as an image of a duck, as well as a hare.
Perception of depth
Illusion of depth perception is the inadequate reflection of a perceived subject and its properties. Nowadays the most explored are the illusory effects which can be observed at the visual perception of two-dimensional contour images. The brain unconsciously sees pictures of single-convex (-concave). Perception depends upon the external direction (real or implied) of the lighting.
Stereo illusions
Stereo pairs overlapped on a periodic structure (Bela Jullecz, Hungary) permit us to observe stereo images the same as like in any other usual stereo pair. The periodic image eases “separation” of the eyes (as a rule infinitely), which after the eyes focus to the distance of several centimeters allows us to see stereo images. This method permits partially combined stereo images by removing the restriction on their size, but overlapping some restrictions on the content of the pictures and practically read by computers.
The illusion of the moon
This visual illusion is characterized by the perceptible size of the planetary body (The moon, Sun) is more (1, 5 times) than when located low above the horizon than the estimated size in the top location of the sky.
Besides given visual psychological illusions, often observed is a physical illusion-“flattening” of the moon (the Sun). The atmosphere changes visible sizes but only in a vertical direction when the sun and Moon are located nearby each other (at great heights the effect is negligible). Refraction at the level of the low edges of their discs (at the horizon) almost by 6 angular minutes the arc is more than the refraction at the level of the top edges. At an angular diameter of the moon’s or Sun’s discs in 30 minutes - 6 minutes is the difference- this is too much. Because the horizontal sizes do not change, so the moon and sun seem flattened at the horizon.
The Eimes room
The room invented by Adelberg Eimes, Jr in 1948 is a model of three dimensional optical illusions. The room is designed in such way that when looking from the front it is normal with perpendicular walls and a ceiling. But in fact, the shape of the room is a trapezium where the distant room is located under the sharp angle of one wall, respectively, at an obtuse angle of other wall. The right angle is, hence, considerably closer to the onlooker than the left angle.
The account is reinforced by the distorted chess squares on the floor and ceiling, a person standing at the closer corner seem gargantuan when compared to the one at the distant corner. When a person crosses corners, the observer feels that he either sharply enlarges, or, on the contrary, shrinks.
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