Hagen–Rubens relation
In optics, the Hagen–Rubens relation (or Hagen–Rubens formula) is a relation between the coefficient of reflection and the conductivity for materials that are good conductors. The relation states that for solids where the contribution of the dielectric constant to the index of refraction is negligible, the reflection coefficient can be written as (in SI Units): R ≈ 1 − 2 2 ϵ 0 ω σ {\displaystyle R\approx 1-2{\sqrt {\frac {2\epsilon _{0}\omega }{\sigma }}}} where ω {\displaystyle \omega } is the frequency of observation, σ {\displaystyle \sigma } is the conductivity, and ϵ 0 {\displaystyle \epsilon _{0}} is the vacuum permittivity.