High-frequency Drude plasma cutoff
= High-frequency Drude plasma cutoff
{title2=$\omega_p^2=ne^2/(m\epsilon_0)$}
The <Drude model> relative permittivity is $1-\omega_p^2/[\omega(\omega+i/\tau)]$. At frequencies large compared with the collision rate but below $\omega_p$, its leading real part is negative and the physical spatial wave-number branch has positive imaginary part. The field therefore decays into the conductor; close to the threshold the full complex response must be retained.