When a beam of light enters a material at an angle, it is bent by an amount determined by the index-of-refraction of the material. All known materials have a positive index-of-refraction; however, it is possible to make an artificial metamaterial that has a negative index-of-refraction. A beam of light will be bent the opposite way in this metamaterial.

Electromagnetic waves interact with the atoms and molecules that compose naturally occurring materials. Materials can therefore be used to guide or manipulate electromagnetic waves–in the way a glass lens can focus light. But the available electromagnetic response from naturally occurring materials is limited; for example, there are no known materials with a negative index-of refraction. Artificial constructed metamaterials, however, are not subject to the same limitations as real materials, and can be used to extend material response.

Materials with negative refractive index have not yet been found or demonstrated using existing materials or compounds. Composite metamaterials, however, such as the one shown to the left, can be engineered to have this unique property. If the elements that compose the metamaterial are very small (in comparison to the wavelength of electromagnetic radiation), then the collection of elements will behave just like a continuous material.