Normal materials have n > 0. Metamaterials engineered with simultaneously negative permittivity (ε < 0) and permeability (μ < 0) achieve negative refractive index.
Snell's Law: n₁ sin(θ₁) = n₂ sin(θ₂)
When n₂ < 0, the refracted ray bends to the same side as the incident ray — a complete reversal! At n₂ = -n₁ a flat slab acts as a perfect lens (Pendry 2000), focusing evanescent waves and overcoming the diffraction limit.