= Solution
At normal incidence from vacuum, the <Fresnel equations> give the <reflectance>
$$
\boxed{R(\omega)=\left|\frac{N(\omega)-1}{N(\omega)+1}\right|^2,\qquad N^2=\epsilon_r.}
$$
A good metal has $R$ close to one below its <plasma frequency>. At very low frequency the electromagnetic field penetrates only a <skin depth>; the local Drude limit gives $1-R\simeq\sqrt{8\epsilon_0\omega/\sigma_0}$. In the collisionless negative-permittivity range the transmitted field is evanescent and reflection remains strong. Near the plasma edge the reflectance drops sharply; above it $\epsilon_r'>0$ and propagating waves become possible. In the ideal free-electron high-frequency limit,
$$
\boxed{R\sim\frac{\omega_p^4}{16\omega^4}\longrightarrow0.}
$$
Finite damping rounds the plasma edge, and interband absorption changes the detailed response of actual metals.
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