# Relationship between the phase and group velocity in a conducting medium

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Is the relation $v_{\varphi }v_{g}=v^{2}=\frac{1}{\mu \varepsilon }$ always true in a plasma ?
Where $v_{\varphi }$, $v_{g}$ are respectively the phase and group velocity of the electromagnetic wave that is propagating in the plasma.

Is the relation $v_{\varphi }v_{g}=v^{2}=\frac{1}{\mu \varepsilon }$ always true in a plasma ?
Where $v_{\varphi }$, $v_{g}$ are respectively the phase and group velocity of the electromagnetic wave that is propagating in the plasma.