A plethora of negative-refraction phenomenons in relativistic and non-relativistic scenarios

In accordance with Snel's law of refraction, whether a plane wave is refracted in the negative sense or positive sense at a planar boundary between two homogenous mediums is determined solely by the orientation of the real parts of the wavevectors involved. Thus, negative refraction should be distinguished from the associated but independent phenomenons of negative phase velocity, counterposition and negative deflection of energy flux. None of these phenomenons is Lorentz covariant.