# Is permittivity/permeability ratio constant or not in media?

1. Sep 19, 2014

### cryptist

We have a constant ratio of them in vacuum. Is it same in a medium also? Or not? And in any case, what is the reason?

2. Sep 20, 2014

### Staff: Mentor

In a medium, $\epsilon$ and $\mu$ are independent of each other, as far as I know. They depend on the atomic/molecular/bulk properties of the material.

Last edited: Sep 20, 2014
3. Sep 20, 2014

### Philip Wood

$\epsilon_r$ (relative permittivity) is usually frequency-dependent, to a greater or lesser degree. A favourite example is water, whose $\epsilon_r$ at very low frequencies is about 80, but at optical frequencies (around $5 \times 10^{14}$ Hz, is about 1.8. This is because water molecules are strongly polarised, and at low frequencies can align themselves strongly with the electric field. At high frequencies they can't flail about (librate?) fast enough to keep pace with the changing field. Except for ferromagnetic materials, $\mu_r$ (relative permeability) is pretty much 1 at all frequencies.