Question

Is negative refraction a real thing or just hypothetical?

Answer 1

Negative refraction is the name for an electromagnetic phenomenon where lightrays are refracted at an interface in the reverse sense to that normally expected. Such an effect can be obtained using a metamaterial which has been designed to achieve a negative value for both (electric) permittivity ε and (magnetic) permeability μ, as in such cases the material can be assigned a negative refractive index. Such materials are sometimes called "double negative" materials.
Negative refraction occurs at interfaces between materials at which one has an ordinary positive phase velocity (i.e. a positive refractive index), and the other has the more exotic negative phase velocity (a negative refractive index).