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Very often in materials physics we are interested in the relative permittivity at optical frequencies, which is usually denoted by $\varepsilon_\text{opt}$ or $\varepsilon_\infty$.

But I'm confused because $\varepsilon\left(\omega=\infty \right)=1$, since at high enough frequencies the medium does not have time to respond to the field and alter the allowed flux per unit charge.

So to me I feel like $\varepsilon_\text{opt}$ is very clear but that $\varepsilon_\infty$ is a stupid convention to denote optical permittivity because it should be =1 always...

But perhaps there is something I'm not understanding? Am I missing something or is it just a convention thing?

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    $\begingroup$ And at x-ray frequencies, the refractive index is even slightly smaller than unity. $\endgroup$ – Pieter Jul 12 '19 at 20:26

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