= Solution
In a simple <metal>, conduction <Electrons> can move collectively in response to an incident <electromagnetic wave>. The <Drude model> gives, with time dependence $e^{-i\omega t}$,
$$
\epsilon_r(\omega)=1-\frac{\omega_p^2}{\omega(\omega+i\gamma)},\qquad
\omega_p^2=\frac{ne^2}{\epsilon_0m^*}.
$$
In the low-loss range $\gamma\ll\omega<\omega_p$, the <relative permittivity> is approximately $1-\omega_p^2/\omega^2<0$. The wavevector inside is predominantly imaginary, so the field is evanescent and penetrates only a short distance. Surface currents reradiate the field; little energy enters a good conductor. The normal-incidence <Fresnel equations> give
$$
\boxed{R=\left|\frac{\sqrt{\epsilon_r}-1}{\sqrt{\epsilon_r}+1}\right|^2\simeq1\quad(\omega<\omega_p,\ \gamma\text{ small}).}
$$
Many ordinary metals have their <plasma frequency> in the ultraviolet, placing visible light below this reflection edge. For $\omega>\omega_p$ the free-carrier contribution permits propagation; at $\omega\gg\omega_p$, $\epsilon_r\to1$ and $R\sim\omega_p^4/(16\omega^4)\to0$. The electrons' inertial response becomes too small to screen the rapidly oscillating field.
\b[Ultraviolet transparency is a simple-metal model prediction, not a universal property of real metals.] <Electronic transitions> between bands and bound-electron absorption can remain strong, shift the plasma edge and prevent transparency; transmission also depends on thickness and attenuation. The robust explanation is the change from a reflective negative-permittivity range to a possible propagating range above the plasma edge.
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