Christoph Hormann <chr### [at] gmxde> wrote:

> Note that according to the most common definitions of the index of
> refraction a negative one is not possible (this has nothing to do with
> physics but mere mathematics).  So if you want to know if POV-Ray
> correctly handles what you call negative ior you first have to decide
> what a negative ior actually is.

The sign of the index of refraction is arbitrary, going strictly by the
mathematics. The physical principle that fixes the sign is conservation of
energy. To conserve energy across a material boundary, normal materials
have to have a positive IOR and these metamaterials have to have a negative
IOR.

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