Invisibility is the state of an object which cannot be seen. An object in this state is said to be Invisible (literally, "not visible"). The term is usually used as a fantasy/science fiction term, where objects are literally made unseeable by magical or technological means; however, its effects can also be seen in the real world, particularly in physics and perceptional psychology.
Since objects can be seen by light in the visible spectrum from a source reflecting off their surfaces and hitting the viewer's eye, the most natural form of invisibility (whether real or fictional) is an object which neither reflects nor absorbs light (that is, it allows light to pass through it). In nature, this is known as transparency, and is seen in many naturally occurring materials (although no naturally occurring material is 100% transparent).
Visibility also depends on the eyes of the observer and/or the instruments used. Thus an object can be classified as "invisible to" a person, animal, instrument, etc. In the research of sensorial perception invisibility has been shown to happen in cycles.
Invisibility is often considered the supreme form of camouflage, as it doesn't show any kind of vital, visual, nor any of the frequencies of the electromagnetic spectrum such as radio, infrared, ultra violet, etc.
People have attributed invisibility to things that are mythical, things that do not exist and are of a religious or supernatural in order to explain why they are not apparent. In the Middle Ages, fern seeds were thought to be invisible since ferns don't have seeds. They were also said to grant invisibility. In medieval astronomy, the crystal spheres that held up the sun, moon, stars, and planets were invisible. Historically, creatures such as goblins and brownies have also been described as invisible or able to become invisible. Currently, many entities or phenomena whose existence is disputed, such as ghosts, demons, qi, and auras, are also ascribed invisibility. In religion, gods, goddesses, angels and demons are commonly thought to be invisible, at least part of the time. Indeed, the omnipresence attributed to the monotheistic God of Abrahamic religions would seem to require invisibility, since otherwise God would be constantly visible to all people because of inhabiting all places, however it has been theorised that a being such as God would exist in dimensions not perceptible to 3-dimensional beings. For example, a 2-dimensional being would only be able to see things in 2 dimensions, length and bredth. Anything outside of these two dimensions, i.e. height, is "invisible" to them. Additionally, Hades, the Greek god of the underworld, possessed a helmet that made the wearer invisible.
- Eugene A. Craig and M. Lichtenstein, "Visibility-Invisibility Cycles as a Function of Stimulus-Orientation," The American Journal of Psychology, 66.4 (Oct., 1953):554-563.
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