Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric field of the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it. In metals, the plasma frequency is in the ultraviolet, and this is why metals are shiny in the visible range. In doped semiconductors, the plasma frequency is usually in the infrared.
Surface plasmons, plasmons near surfaces, interact strongly with light, resulting in a polariton. They play a role in surface-enhanced Raman scattering and in explaining anomalies in diffraction from metal gratings, among other things.
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