Invisibility is the state of an object that can not be seen. An object in this state is said to be invisible (literally, "not visible"). The term is often used in fantasy / science fiction, where objects can not be seen by magical or technological means; however, their effects can also be demonstrated in the real world, particularly in the physics and perceptual psychology classes.
Since objects can be seen by light in the visible spectrum from a source that reflects on their surfaces and strikes the eye of the viewer, the most natural form of invisibility (either real or fictional) is an object that does not reflect absorbs light (that is, allows light to pass through). This is known as transparency, and is seen in many natural materials (although no natural material is 100% transparent).
The perception of invisibility depends on several optical and visual factors. For example, invisibility depends on the eyes of the observer and / or instruments used. Thus, an object can be classified as "invisible to" a person, an animal, an instrument, etc. In research on sensory perception it has been shown that invisibility is perceived in cycles.
Invisibility is often regarded as the supreme form of camouflage, since it does not reveal to the viewer any vital signs, visual effects or any frequency of the electromagnetic spectrum detectable to the human eye, rather than using radio, infrared or ultraviolet. In the view of illusion, invisibility is a special case of illusion effects: the illusion of free space.