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Illusions

illusion, a misrepresentation of a “real” sensory stimulus—that is, an interpretation that contradicts objective “reality” as defined by general agreement. For example, a child who perceives tree branches at night as if they are goblins may be said to be having an illusion. An illusion is distinguished from a hallucination, an experience that seems to originate without an external source of stimulation. Neither experience is necessarily a sign of psychiatric disturbance, and both are regularly and consistently reported by virtually everyone.

Optical phenomena


optical illusion: refraction of light
Numerous optical illusions are produced by the refraction (bending) of light as it passes through one substance to another in which the speed of light is significantly different. A ray of light passing from one transparent medium (air) to another (water) is bent as it emerges. Thus, the pencil standing in water seems broken at the surface where the air and water meet; in the same way, a partially submerged log in the water of a swamp gives the illusion of being bent.

Sensory illusions
Many sensory illusions may be described as the aftereffects of the stimulation, or overstimulation, of the senses. Sensitivity in any of the senses may be measured as the just-perceptible intensity (threshold, or limen) of the appropriate stimulus. The smallest detectable stimulus is called the absolute threshold, while the smallest detectable change in the intensity of a stimulus is called the difference threshold. Such thresholds can serve as points of reference, or anchors, against which subsequent stimuli are judged or perceived. Yet sensory anchors fluctuate within the same individual under different conditions, and in some cases they can mislead a person about the properties of subsequent stimuli. For example, two successive stimuli may be identical but nevertheless give the illusion of being different.

Colour illusions

The normal human eye can detect about 130 gradations of colour in the visible spectrum (as in the rainbow), about 20 barely noticeable differences within a given colour, and about 500 variations of brightness. However, when two spots of equally bright light are observed in close succession, the first intensity may seem brighter. The first light may be said to serve the function of brightness adaptation (or adjustment) in the eye; therefore, the second light will fall on a partly adapted and therefore less sensitive retina. In a brief time, such excitement in the retina (or even in the brain) tends to subside, or fade. As a result of the fading traces of excitement, various hues of a given colour may appear to be lighter or darker when looked at successively.

Fantasy
Imagination
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Reflection
mind
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brain
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