Just a bit more info on invisibility:

Invisibility is the state of an object which cannot be seen. 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 nor absorbs light (that is, it 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.

People have attributed invisibility to things that do not exist in order to explain why they're not apparent. In the Middle Ages, fern seeds were thought to be invisible since ferns don't have seeds. They were also said to grant invisibility. In medieval astronomy, the crystal spheres that held up the sun, moon, stars, and planets were invisible. Fairies, gnomes, brownies etc have also been described as invisible or able to become invisible.
Currently, many entities or phenomena whose existence is disputed, such as ghosts, demons, qi, and auras, are also ascribed invisibility.

Philadelphia Experiment

The Philadelphia Experiment was a supposed secret experiment conducted by the U.S. Navy at the Philadelphia Naval Yards at Philadelphia, Pennsylvania, on or before October 28, 1943, which went horribly awry. The experiment was allegedly to cast a veil of radar and possibly optical invisibility or transparency around a ship. Though most mainstream experts argue the incident is an urban legend, a number of people argue that the Philadelphia Experiment could be genuine. Whether true or not, the Philadelphia Experiment has had a major ripple effect on conspiracy theory, and elements of the Philadelphia Experiment feature in many other conspiracy tales and provided the basis for various films and mini-series.