How Light Bends
The nature of light has always been a puzzling concept for physicists. Light exhibits the properties of both waves and particles. It travels in straight lines, like an unaccelerated particle; yet, it exhibits diffraction patterns as do water and sound waves. This dichotomy is called the wave-particle duality. It is the wave properties of light that make cloaking possible. When light interacts with an object (a wall, mirror, or even air), it reflects and refracts. Light reflects from a mirror like a ball bouncing off a wall: the incident and reflected rays make equal angles with the normal line (perpendicular line) to the reflecting surface. We can use this understanding to direct light along precise paths. Lasers and holograms make use of this simple law of reflection. It is the more complicated law of refraction, however, that describes the bending of light. Recent advances in the understanding of this rule allow for the prospect of engineered invisibility. Bending light around what is usually blocked by an object, without reflection, would allow the viewer to see what is behind the object instead of the object itself, thus creating the illusion of invisibility (see Fig. 1).
The laws of physics govern the bending of light, and an engineer can only make light go where the laws of physics allow. The rule that governs the path of light rays is Snell’s Law, which says that light travelling through a single material (e.g. air, water, glass) travels in straight lines, and when light travels from one material into another, the rays bends (see Fig. 2). Snell’s Law provides a specific formula for how much light bends in terms of the two media’s indices of refraction, a property specific to each substance.
Mathematics of Invisibility
In 2000, electrical engineers at Duke University confirmed Vesalago’s theory by synthesizing electromagnetic metamaterials (see Fig. 3) with negative indices of refraction and thus jump-starting research in artificial invisibility. David Smith, an electrical engineer at Duke, has been acknowledged as the first man to create a material that could cloak an object from an electromagnetic wave. His device, however, does not cloak an object from visible light, but from microwaves, which have much longer wavelengths (on the scale centimeters) than those of the visible spectrum (on the scale of nanometers). The cloak is an arrangement of loops comprised mostly of plastic encased in tiny copper rings . As described in Scientific American, “a central copper ring – the object to be cloaked – is surrounded by concentric rings of metamaterial standing one centimeter tall and spanning 12 centimeters. The rings are sandwiched between two plates so that microwaves can only travel through the cloak in the plane of the rings” . The dimensions of the loops and rings are smaller than the wavelengths of the waves being cloaked, a property that is characteristic of all metamaterials. In order for the cloak to correctly bend the light wave, the material properties must vary substantially within the distance of a single wavelength. By creating a negative index of refraction, Smith and his colleagues manipulated microwaves to interact with the rings, moving them around the center and refracting back to the other side at the same angle at which they approached the object . Smith explains that, had this been visible light, the deviations from the theoretical predictions would only cause the viewer to see the background with slightly less intensity, while any object in the center would still have been invisible.
In order to make something invisible to the naked eye, all wavelengths of visible light must be refracted around the object simultaneously. The visible spectrum ranges from 400-790 THz , as can be seen again in Fig. 4. As of now, metamaterials only work for a specific wavelength of light, and designing the materials to defer the entire spectrum of frequencies will require new insights.
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