Abstract
In 1968, Veselago theoretically pioneered a new kind of electromagnetic materials with simultaneously negative permittivity and permeability, called Veselago materials here. Both theoretical simulations and experimental evidences have verified the irrefutable fact of negative refraction. A widely spread opinion is that the refractive index of a Veselago material must be negative in the literature. Here we argue this opinion, and present the concept of the refractive index of energy flow and the analogue Snell's law of energy flow. We reveal the underlying physics behind negative refraction in the Veselago material, indicating that the refractive index is positive while the refractive index of energy flow is indeed negative.