Most materials have the refractive index, n lt 1. So, when a light ray from air enters a naturally occuring material, then by Snell's law, (sin theta_(1))/(sin theta_(2)) = (n_(2))/(n_(1)), it is understood that the refracted ray bends towards the normal. But it never emerges on the same side of the normal as the incident ray. According to electromagnetism, the refractive index of the medium is given by the relation, n=((c)/(V))=pmsqrt(epsilon_(r)u_(r)), where c is the speed of electromagnetic waves is vacuum, v its speed in the medium , epsilon_(r) and mu_(r) are relative permittivity and permeability of the medium respectively. In normal materials, both epsilon_(r) and mu_(r) are positive, implying positive n for the medium. When both epsilon_(r) and mu_(r) are negative, one must choose the negative root of n. Such negative refractive index material can now be artificially prepared and are called metamaterials. They exhibit significantly different optical behaviour, without violating any physical laws. Since n is negative, it results in a change in the direction of proporgation of the refracted light. However, similar to normal materials, the frequency of light remais unchanged upon refraction evenin metal - materials.