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.
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 film making, people, objects, or backgrounds can be made to look invisible on-camera through a process known as blue screen (or green screen).