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 unseeable 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).

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 by Environment

An object may be classified as "invisible" if it cannot be seen due to environmental factors other than the fact that it doesn't reflect light. An object that might normally be seeable may be classified as invisible if it is:

• Behind an object.
• The same colour or pattern as the background. (Camouflage)
• Patterned so that its outline is hard to determine.
• In an environment which is too dark or too bright.
• Not in a particular observer's line-of-sight. (Especially when driving)
• Transparent. (air and many other gases)

Invisibility in Physics

Theoretical and practical physics offer several causes of invisibility. An object may be invisible if it is:

• So massive that its escape velocity exceeds the speed of light (e.g. a black hole)
• Emitting or reflecting light outside the wavelength range of visible light. (Radiation is generally invisible by this means). Unfortunately, this would result in any obscured human being becoming not invisible and transparent, but completely opaque and resembling a human-shaped black hole.
• So tiny that it cannot be detected by an observer.

Invisibility by Technology

Technology can be used theoretically or practically to render real-world objects invisible:

• Making use of real-time image displayed on a wearable display, scientists are able to create a see-through effect, if not invisibility. This is known as optical camouflage and has been used in many science fiction works.
• It has been speculated that an object could appear invisible if light were bent around it.
• Invisibility (lower visibility) for radar is called stealth technology.
• Often a hypothetical "cloaking device" can be used to make objects invisible.
• In filmmaking, people, objects, or backgrounds can be made to look invisible on-camera through a process known as bluescreen (or greenscreen).

Invisibility by Magic

Invisibility, usually by magic, is a popular theme in fantasy, mythology and Ceremonial magic. It is often used as a gameplay device in role-playing and strategy games. Typically, 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.
• Magical potions can be consumed to grant temporary invisibility.
• Magic spells can be cast on people or objects, usually giving temporary invisibility.
• Some mystical creatures can make themselves invisible at will, such as Chinese dragons in some tales, which can shrink so small that humans cannot see.
• An invisibility ritual part of one of the manuscript of the Hermetic Order of the Golden Dawn as it appears on the Book "The Golden Dawn" transliterated by Israel Regardie.

Where magical invisibility is concerned, the issue may arise of whether or not 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.

Examples of Invisibility in Fiction

The idea of being unseen and hence undetectable has fascinated mankind for generations. This concept of invisibility has been explored in many myths, novels, movies and video games, some of them comedies.

• Mythology and folklore
  • In many myths and legends, gods, spirits, fairies, angels, and demons are often invisible or can choose to become invisible at will.
  • The ring of Gyges is described in a story in Plato's The Republic. A peasant finds a ring in the tomb of a dead king which allows him to become invisible at will. Plato has him enter the palace, seduce the queen, and plot to kill the king, arguing that power, such as this, corrupts absolutely.
  • The hero Perseus went equipped with a helmet of invisibility to kill Medusa.
  • A magic cloak, made by Alberich the dwarf, granted invisibility to Sigurd.
  • In German fairy tales, magical caps called tarnkappes are worn by dwarfs. The caps can make an entire village of dwarfs invisible.
• Modern fiction
  • The Invisible Man (1897) by H. G. Wells is a well-known novel about invisibility, later made into a film and several TV series.
  • One function of the One Ring in J. R. R. Tolkien's The Lord of the Rings series was to render the user invisible. Unfortunately, it had an evil influence with negative effects on the wearer's actions.
  • The Philadelphia Experiment (1984) is a movie about an attempt to make a ship invisible.
  • The film Predator and animated television series like Batman Beyond and Max Steel depict a a kind of stealth mode that is a partial invisibility where the subject is largely transparent and/or translucent. While a concentrated look can spot it, it still makes the wearer extremely hard to see which eases stealth movement. Such a suit may actually be possible (see External links below).
  • The Harry Potter series of novels and movies featured a cloak of invisibility that, when worn, makes the covered body parts, including the clothing beneath the cloak invisible, the cloak itself and perhaps also the contents of the pockets, invisible. The cloak does not stop one from being solid or making noise. When one wears an Invisibility Cloak in a crowd, the wearer has to take great care to not knock into anyone, which leads to awkward questions.
  • In comic books, there are superheroes such as the Invisible Woman (who can bend light around herself without distortion) that have the power to become invisible at will as well as wizards like Doctor Strange who have invisibility spells in their possession.
  • In the film Mystery Men (1999), the "Invisible Boy" is invisible when nobody is looking at him.
  • In the film Hollow Man (2000), an injection is tested on animals (and eventually, humans) which made the organism invisible, organ by organ. Unfortunately, the serum also causes serious side effects.
  • In The Hitchhiker's Guide to the Galaxy "trilogy" of novels by Douglas Adams, the Somebody Else's Problem field is a humorous concept of a field which makes people believe the object in question is "somebody else's problem" and therefore do not see it.
  • In the Star Trek universe, some ships of the Klingon Empire and the Romulan Star Empire, as well as the Federation ship USS Defiant, are equipped with pieces of hardware known as cloaking devices that conceal them from most varieties of scans, including visual.
  • Cloaking technology is also employed by large battleships in the game Colony Wars.
  • In Metal Gear Solid, Otacon wore a stealth camoflauge prototype while sneaking around the facility to provide you help. Ninja also used a similar suit. The Fear used it in Metal Gear Solid 3: Snake Eater as well. It is also an unlockable item that the player can use to thwart his enemies.
  • Blizzard Entertainment's strategy games have always had invisibility as a gameplay device. For example, in StarCraft, many units employ cloaking devices, while in Warcraft III, units employ a range of invisibility spells and camouflage at night time.
  • In The Incredibles, one of the family members, Violet Parr (Vi) has the ability to render her invisible to people. The ability is for her body only. A suit is made for her that can also go invisible when she does.
  • The short story "They Go Bump" by David Barr Kirtley is about a group of soldiers who wear invisibility suits. The main character has trouble walking (not being able to see his feet) and starts to wonder if his invisible companions are really who they say they are.
  • The videogame, Psychonauts features invisibility as one of many psychic powers unlocked and made available to the player throughout the game. The power can be used to escape enemies, and it particularly useful during boss encounters.
  • In the book series Artemis Fowl, fairies can vibrate at a very high frequency, making them invisible all creatures except rats and two species of monkeys.