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 Optical Illusions

Optical illusions occur as a result of how information is received through the eyes and how that information is interpreted in the brain.

When light enters the eyes, it passes first through the cornea, a transparent cover, and then through a watery fluid called the aqueous humor, and through the pupil, which is an opening in the colored part of the eye called the iris. The size of the pupil increases when there is less light and decreases when there is more.

Light next passes through the lens, which focuses the light through a transparent jell, called the vitreous body, onto the retina. The retina has two types of light sensitive cells, rods and cones. Rods are particularly sensitive to shades of light and outlines, and they are important in both night and peripheral vision. Cones have specialized pigments that are sensitive to either red, green, or blue. Cones enable people to see details. Rods and cones cover the entire retina except for a small area just above the optic nerve, known as the blind spot.

When a person looks at an object of a particular color for a long time and then suddenly looks at a blank space, the person sees a ghostly outline of the object in its complimentary color. If the object is red, the person sees green. If the object is blue, the person sees yellow. This happens because the cones are sensitive in pairs. When a cone sensitive to one color is turned off, the other color is briefly turned on.


How information is perceived depends also on learned perceptions. For example, people who have spent their lives in a dense forest have difficulty judging distance and size in large open areas like the prairies. They do not realize that objects that are large may appear small because they are far away. Although their vision is sharp, their perception of the world is inaccurate. What they have learned about interpreting the information provided by their senses is incomplete. People's perceptions can be fooled when seeing something that lies outside their previous experience.

When looking at optical illusions, the eyes are recording an image that the brain misinterprets. The brain likes to group things into patterns of familiar forms. It is always comparing things with its experience and knowledge of the world. For example, the brain knows that a bicycle wheel is circular. So even though the wheel may not look circular from a particular angle or perspective, the brain interprets the wheel as circular.

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