Question

What are the conditions for an object to be “invisible”?

Answer 1

An object is "invisible" if all light rays traveling from behind the object to the viewer's eye are undeflected. In other words, they travel from the background to your eye exactly the same way when the object is present and when the object is absent.

This means that there can be no absorption of the light rays, and that if any light rays are deflected by the object, the object must also "return them" to their original paths. Otherwise, you'd be able to tell that a light ray from the background had been deflected (or was "missing"), which would allow you to detect the object. For an object made of a single material, with a single index of refraction, this turns out to imply that the object must have the same index of refraction as the medium the rays are traveling through. In other words, you must haveboth no absorption and no refraction for such an object to be "invisible".

However, it is possible to (effectively) vary the index of refraction in various locations in the object to "guide" the light around an object; this is the science behind metamaterial cloaking. So far, this has only been achieved for microwave frequencies (rather than for visible light), but physicists are continuing to work in this field and see whether they can extend it into the visible range.

ñ

. To become invisible, anobject must do two things: it has to be able to bend light around itself, so that it casts no shadow, and it must produce no reflection.