# Hagen–Rubens relation

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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.[1] 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):[2] $R\approx1-2\sqrt{\frac{2\epsilon_0\omega}{\sigma}}$
where $\omega$ is the frequency of observation, $\sigma$ is the conductivity, and $\epsilon_0$ is the vacuum permittivity.