= Solution
The mobile electrons in a <metal> screen an incident optical field. In the weak-damping <Drude model>, $\varepsilon(\omega)\simeq\varepsilon_\infty-\omega_p^2/\omega^2$. Below the screened <plasma frequency>, the real dielectric response is negative, the wave is evanescent inside the metal, and a clean surface is highly reflective. Above the plasma edge, the response can become positive and propagating waves are allowed, explaining the qualitative transmission window at sufficiently high, often ultraviolet, frequencies.
This is a free-carrier argument, not a statement that every metal is transparent throughout the ultraviolet. <Interband absorption>, damping and sample thickness can still produce strong absorption above the plasma edge.
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