eli5: What is a negative-index metamaterial
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Generally, a metamaterial is composed of several different materials arranged in a certain way to get certain desirable properties on a macro scale. This arrangement is called a unit cell and is the basic building block of the meta material.
Assuming you are referring to a meta material with a negative index of refraction, this is an arrangement of unit cells that produce a material with a negative index of refraction.
Naturally occurring material will bend the light traveling through it (this is called refraction) because the speed of light is different in different materials. How much this light will be bent depends on the angle the light hits the material (or medium), the absolute refractive index of the material. The refractive index is dependent on the wavelength of the light, and is typically found experimentally. This bending actually happens at the boundary between media, and the relationship between the angle the light enters and leaves the boundary is called Snells law.
A negative index of refraction behaves in ways that are opposite to most traditional example of refraction such as in water or glass. When light crosses the boundary of a material with a negative index of refraction the propagation of the light is reversed. [This does some wacky things](https://en.m.wikipedia.org/wiki/File:Negative_refraction_index_focusing.png). Typically when light is refracted in the boundary of 2 positive index of refraction media, it will appear on the opposite side of the normal line (see case a). In the boundary between a positive index of refraction and negative index of refraction media, the light will appear on the same side of the normal as it entered (see case b). Because of these wacky properties, scientists can try to make lenses that can see details beyond a current diffraction limit that current lenses in the visible spectrum are capped by.
There is a lot more complicated stuff happening with these and I’m worried I’ve oversimplified. Wikipedia can be quite the rabbit hole, if you want to learn more about this stuff I think you’ll need to start looking at some upper level college physics.
[Wikipedia](https://en.m.wikipedia.org/wiki/Negative-index_metamaterial)
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