Something is invisible if it can not be seen. Possible causes are:

• too small
• behind something else
• transparent
• the same as the background
• too dark
• be coloured in a pattern that makes the outline hard to determine
• so massive that its escape velocity exceeds the speed of light (e.g., a black hole)

Radiation is generally invisible outside the wavelength range of visible light.

Visibility also depends on the eyes of the observer and/or the instruments used. Thus one can say "invisible to" and then a person, animal, instrument, etc.

Invisibility can be achieved, or at least approximated, by camouflage.

Making use of real-time image displayed on a wearable display, scientists are able to create a see-through effect, if not invisibility.

Invisibility (lower visibility) for radar is called stealth technology.

Rings, cloaks, other devices and potions that render the wearer invisible have long been featured in myths, fairy tales and role playing games. The concept of invisibility has also been explored in several movies, many of them comedies.

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.

According to physics a truly invisible man should be blind, as the eye works by absorbing photons while transparency would dictate that none would be absorbed. If however the light could somehow be retransmitted with the same properties behind the eye then this could be overcome.

Examples of invisibility devices in fiction:

• In myths and legends, spirits, fairies, angels, and demons are often invisible.
• The ring of Gyges, described in a story in Plato's 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.
• The Invisible Man (1897) by H. G. Wells is a well-known novel about invisibility.
• 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, a project to make a ship invisible.
• The movie serial Predator depicts an alien race that achieves invisibility by use of a special kind of suit. Such a suit should really be possible, see external links below.
• The Harry Potter series featured a cloak of invisibility that, when worn, makes the covered body parts, including the clothing beneath the cloak, 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. Harry has a Cloak that he inherited from his father. Also, another character owned one (this person's identity is a spoiler.)
• In comic books, there are superheroes like 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.
• Douglas Adams proposed the Somebody Else's Problem field in the Hitchhiker's Guide to the Galaxy.
• 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.
• In Metal Gear Solid, Otacon wore a stealth camoflauge prototype while sneaking around the facility to provide you help. 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.

External links

• info on "stealth suit" (http://www.guardian.co.uk/Print/0,3858,4199601,00.html)
• Some examples (http://members.aol.com/doder1/invisib1.htm)
• Such a suit really made (http://www.cnn.com/2003/TECH/02/07/japan.invisible.ap) Note: the picture was taken by a special camera that displays images behind the wearer of the suit. The suit itself does not render invisibility. In essence, the suit functions as a bluescreen onto which the image behind the suit can be displayed through camera.