Editing Refractive index (section)

===Wave impedance===
{{see also|Wave impedance}}
The wave impedance of a plane electromagnetic wave in a non-conductive medium is given by
<math display="block">\begin{align}
Z &= \sqrt{\frac{\mu}{\varepsilon}} = \sqrt{\frac{\mu_\mathrm{0}\mu_\mathrm{r}}{\varepsilon_\mathrm{0}\varepsilon_\mathrm{r}}} = \sqrt{\frac{\mu_\mathrm{0}}{\varepsilon_\mathrm{0}}}\sqrt{\frac{\mu_\mathrm{r}}{\varepsilon_\mathrm{r}}} \\
  &= Z_0 \sqrt{\frac{\mu_\mathrm{r}}{\varepsilon_\mathrm{r}}} \\
  &= Z_0 \frac{\mu_\mathrm{r}}{n}
\end{align}</math>

where {{math|''Z''{{sub|0}}}} is the vacuum wave impedance, {{mvar|μ}} and {{mvar|ε}} are the absolute permeability and permittivity of the medium, {{math|''ε''{{sub|r}}}} is the material's [[relative permittivity]], and {{math|''μ''{{sub|r}}}} is its [[Permeability (electromagnetism)|relative permeability]].

In non-magnetic media (that is, in materials with {{math|''μ''{{sub|r}} {{=}} 1}}), <math>Z = {Z_0 \over n}</math> and <math>n = {Z_0 \over Z}\,.</math>

Thus refractive index in a non-magnetic media is the ratio of the vacuum wave impedance to the wave impedance of the medium.

The reflectivity {{math|''R''{{sub|0}}}} between two media can thus be expressed both by the wave impedances and the refractive indices as
<math display="block">\begin{align}
R_0 &= \left| \frac{n_1 - n_2}{n_1 + n_2} \right|^2 \\
    &= \left| \frac{Z_2 - Z_1}{Z_2 + Z_1} \right|^2\,.
\end{align}</math>