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 phenomenon is studied by physics and perceptual psychology.
Since objects can be seen by light from a source reflecting off their surfaces and hitting the viewer's eyes, 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).
Invisibility perception depends on several optical and visual factors. For example, invisibility 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 research on sensorial perception it has been shown that invisibility is perceived in cycles.
Invisibility is often considered to be the supreme form of camouflage, as it does not reveal to the viewer any kind of vital signs, visual effects, or any frequencies of the electromagnetic spectrum detectable to the human eye, instead making use of radio, infrared or ultraviolet wavelengths.
In illusion optics, invisibility is a special case of illusion effects: the illusion of free space.
The term is often used in fantasy and science fiction, where objects cannot be seen by means of magic or hypothetical technology.
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Practical efforts
Technology can be used theoretically or practically to render real-world objects invisible.
Making use of a 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 declared to be invisible to radar, all officially disclosed applications of the technology can only reduce the size and/or clarity of the signature detected by radar.
In 2003 the Chilean scientist Gunther Uhlmann postulated the first mathematical equations to create invisible materials.
In 2006, a team effort of researchers from Britain and the US announced the development of a real cloak of invisibility, an artificially made meta material that is invisible to the microwave spectrum, 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.
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 predicted to be applied to radio waves within five years, and the distortion of visible light is an eventual possibility. The theory that light waves can be acted upon the same way as radio waves is now a popular idea among scientists. The agent can be compared to a stone in a river, around which water passes, but slightly down-stream leaves no trace of the stone. Comparing light waves to the water, and whatever object that is being "cloaked" to the stone, the goal 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.
Psychological
A person can be described as invisible if others refuse to see them or routinely 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 the author, being overlooked on account of his status as an African American. This is supported by the quote taken from the Prologue, "I am invisible, understand, simply because people refuse to see me." (Prologue.1)
Fictional use
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 the object is taken off).
Magical potions can be consumed to grant temporary or permanent 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 in some tales in which leprechauns or Chinese dragons can shrink so much that humans cannot see them.
In science fiction, the idea of a "cloaking device".'
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 worn by and any items carried by the invisible being are also rendered invisible. In general they are also regarded as being invisible, but in some instances clothing remains visible and must be removed for the full invisibility effect.