  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.
