I've read that in some materials, $\mu$ can be a tensor, not a mere scalar.   (I haven't actually dealt with tensors before, but I'm assuming for my purposes here, it is synonymous with "matrix".)

I'm not sure if the same holds for $\epsilon$, but I'm assuming it might there too.

I'm fine with either of those, but then I'm wondering what [the formula](https://www.doitpoms.ac.uk/tlplib/dielectrics/dielectric_refractive_index.php)

$$\epsilon\,\mu = \frac{1}{c_m^2}$$

turns into when dealing with tensors? ($c_m$ being the speed of light in the medium)