Invisibility refers to 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 useable by magical or technological means. However, its effects can also be seen in the real world, particularly in physics.
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 does not reflect light (that is - 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).
The concept of a cloaking device, or the invisibility cloak, is to steer light waves around an object to make it appear as if nothing were there
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.
Invisibility has been on humanity?s wish list at least since Amun-Ra, a deity who could disappear and reappear at will, joined the Egyptian pantheon in the earlier BC. With recent advances in optics and computing, however, this elusive goal is no longer purely imaginary. Susumu Tachi, an engineering professor at the University of Tokyo, demonstrated a crude invisibility cloak. Through the clever application of some dirt-cheap technology, the Japanese inventor has brought personal invisibility a step closer to reality.
????????? Tachi?s cloak- a shiny raincoat that serves as a movie screen, showing imagery from a video camera positioned behind the wearer- is more gimmick than practical prototype. Nonetheless, from the right angle and under controlled circumstances, it does make a sort of ghost of the wearer. In addition, unlike traditional camouflage, it is most effective when either the wearer or the background is moving (but not both).
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