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Invisibility is the state of an object that cannot be seen. An object in this state is said to be invisible (literally, "not visible"). The term is often used in fantasy/science fiction, 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 perceptual 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 that neither reflects nor absorbs light (that is, it allows light to pass through it). 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, ultraviolet, etc.
Practical efforts 
Technology can be used theoretically or practically to render real-world objects invisible:
- Making use of real-time image displayed on a wearable display, it is possible to create a see-through effect. This is known as active camouflage.
- Though stealth technology is cited as invisibility to radar, all officially disclosed applications of the technology can only reduce the size and/or clarity of the signature detected by radar.
- In some science fiction stories, a hypothetical "cloaking device" is used to make objects invisible. On Thursday, October 19, 2006 a team effort of researchers from Britain and the US announced the development of a real cloak of invisibility, though it is only in its first stages.
- In filmmaking, people, objects, or backgrounds can be made to look invisible on camera through a process known as chroma keying.
- An artificially made meta material that is invisible on the microwave light spectrum.
Engineers and scientists have performed various kinds of research to investigate the possibility of finding ways to create real optical invisibility (cloaks) for objects. Methods are typically based on implementing the theoretical techniques of transformation optics, which have given rise to several theories of cloaking.
- Currently, a practical cloaking device does not exist. A 2006 theoretical work predicts that the imperfections are minor, and metamaterials may make real-life "cloaking devices" practical. The technique is suspected to be applied to radio waves within five years, and eventually visible light is a possibility. The theory that light waves can be acted upon the same way as radio waves is now a popular idea among scientists and can be compared to a stone in a river, in which the water passes around it, but leaves no trace of a stone being in the water slightly down-stream. Comparing light waves to the water and whatever object that is being "cloaked" to the stone, the desire is to have light waves pass around that object, leaving no visible aspects of it, possibly not even a shadow. This is the technique depicted in the 2000 television portrayal of The Invisible Man.
- Two teams of scientists worked separately to create two "Invisibility Cloaks" from 'metamaterials' engineered at the nanoscale level. They demonstrated for the first time the possibility of cloaking 3-dimensional (3-D) objects with artificially engineered materials that redirect radar, light or other waves around an object. While one uses a type of fishnet of metal layers to reverse the direction of light, the other uses tiny silver wires. Xiang Zhang, of the University of California, Berkeley said: "In the case of invisibility cloaks or shields, the material would need to curve light waves completely around the object like a river flowing around a rock. An observer looking at the cloaked object would then see light from behind it, making it seem to disappear."
- UC Berkeley Researcher Jason Valentine's team made a material that affects light near the visible spectrum, in a region used in fibre optics: 'Instead of the fish appearing to be slightly ahead of where it is in the water, it would actually appear to be above the water's surface. It's kind of weird. For a metamaterial to produce negative refraction, it must have a structural array smaller than the wavelength of the electromagnetic radiation being used." Valentine's team created their 'fishnet' material by stacking silver and metal dielectric layers on top of each other and then punched through with holes. The other team used an oxide template and grew silver nanowires inside porous aluminum oxide at tiny distances apart, smaller than the wavelength of visible light. This material refracts visible light.
- The Imperial College London research achieved results with microwaves. An invisibility cloak layout of a copper cylinder was produced in May, 2008, by physicist Professor Sir John Pendry. Scientists working with him at Duke University in the US put the idea into practice.
- Pendry, who theorized the invisibility cloak "as a joke" to illustrate the potential of metamaterials, said in an interview in August 2011 that grand, theatrical manifestations of his idea are probably overblown: "I think it’s pretty sure that any cloak that Harry Potter would recognize is not on the table. You could dream up some theory, but the very practicality of making it would be so impossible. But can you hide things from light? Yes. Can you hide things which are a few centimeters across? Yes. Is the cloak really flexible and flappy? No. Will it ever be? No. So you can do quite a lot of things, but there are limitations. There are going to be some disappointed kids around, but there might be a few people in industry who are very grateful for it."
A person can be described as invisible if others refuse to see them, or overlook them. The term was used in this manner in the title of the book Invisible Man, by Ralph Ellison, in reference to the protagonist, likely modeled after himself, being overlooked on account of his status as an African American.
Fictional use 
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In fiction, people or objects can be rendered completely invisible by several means:
- Magical objects such as rings, cloaks and amulets can be worn to grant the wearer permanent invisibility (or temporary invisibility until object is taken off).
- Magical potions can be consumed to grant temporary invisibility.
- Magic spells can be cast on people or objects, usually giving temporary invisibility.
- Some mythical creatures can make themselves invisible at will, such as some versions of Leprechaun, and Chinese dragons in some tales, which can shrink so small that humans cannot see them.
In some works, the power of magic creates an effective means of invisibility by distracting anyone who might notice the character, but since the character is not truly invisible, the effect could be betrayed by mirrors or other reflective surfaces.
Where magical invisibility is concerned, the issue may arise of whether the clothing and items carried by the invisible wearer/carrier are also rendered invisible. In general, they are, but in some instances, clothing remains visible and must be removed for the full invisibility effect.
See also 
- Active camouflage
- Cloak of invisibility
- Cloaking device
- Immortal, Invisible, God Only Wise
- Somebody Else's Problem
- 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.
- Cloak of invisibility: Fact or fiction? - Innovation - MSNBC.com
- Nachman, Adrian I. (November 1988). "Reconstructions From Boundary Measurements". Annals of Mathematics (Annals of Mathematics) 128 (3): 531–576. doi:10.2307/1971435. JSTOR 1971435.
- Wolf, Emil; Tarek Habashy (May 1993). "Invisible Bodies and Uniqueness of the Inverse Scattering Problem". Journal of Modern Optics 40 (5): 785–792. Bibcode:1993JMOp...40..785W. doi:10.1080/09500349314550821. Retrieved 2006-08-01.
- Pendry, J. B.; D. Schurig, and D. R. Smith (June 2006). "Controlling Electromagnetic Fields". Science 312 (5781): 1780−1782. Bibcode:2006Sci...312.1780P. doi:10.1126/science.1125907. PMID 16728597. Retrieved 2006-08-01.
- Leonhardt, Ulf (June 2006). "Optical Conformal Mapping". Science 312 (5781): 1777–1780. Bibcode:2006Sci...312.1777L. doi:10.1126/science.1126493. PMID 16728596. Retrieved 2006-08-01.
- Cho, Adrian (2006-05-26). "High-Tech Materials Could Render Objects Invisible". Science. p. 1120. Retrieved 2006-08-01.
- "Invisibility cloak a step closer as scientists bend light 'the wrong way'". Daily Mail (London). 2008-08-11.
- themoneytimes.com,Scientists Turn Fiction Into Reality, Closer to Make Objects "Invisible"
- mirror.co.uk, Secrets of invisibility discovered
- John Pendry video: The birth and promise of metamaterials, SPIE Newsroom, 18 October 2011, doi:10.1117/2.3201110.02.
- The Digital Chameleon Principle: Computing Invisibility by Rendering Transparency
- Physics World special issue on invisiblity science - July 2011
- Light Fantastic: Flirting With Invisibility - The New York Times
- Invisibility in the real world Interesting picture of a test tube's bottom half invisible in cooking oil.
- Brief piece on why visible light is visible - Straight Dope
- CNN.com - Science reveals secrets of invisibility - Aug 9, 2006