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Relative permittivity shows that force of interaction in some media is 3 times less than in vacuum ($\varepsilon = 3$) for example. When looking tables of permittivity I never saw a value less than 1.

Russian wikipedia says that $\varepsilon > 1$ is typical for low frequency. So I guess that under some conditions it is possible that $\varepsilon < 1$.

  1. For a static (electrostatic) electrical field is it ever possible (and for what material) that $\varepsilon < 1$?

  2. For the electric component of EMF field - when is it possible that $\varepsilon < 1$?

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    $\begingroup$ $\epsilon < 1$ for most materials at extreme ultraviolet and x-ray wavelengths. Also, metamaterials frequently have $\epsilon < 1$ at IR and visible wavelengths. $\endgroup$
    – garyp
    Commented Jun 11, 2018 at 20:00

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Yes this is quite common in metals at optical frequencies. For silver at 600 nm the real part of epsilon is -16.

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    $\begingroup$ And what exactly would it mean if we use that as dielectric in parallel plate capacitor ? $\endgroup$
    – frakod
    Commented Nov 27, 2021 at 16:06
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Yes,refractive index of ionospheric plasma is less than 1 Refractive index equals square root of relative permittivity so for inospheric plasma it is less than 1

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