[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] where
For metals, this relation holds for frequencies (much) smaller than the Drude relaxation rate, and in this case the otherwise frequency-dependent conductivity
can be assumed frequency-independent and equal to the dc conductivity.
The relation is named after German physicists Ernst Bessel Hagen and Heinrich Rubens who discovered it in 1903.
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