This talk is about the infrared (mostly far-infrared) properties of cuprate superconductors. When viewed at frequencies below about 8 THz (250 cm-1; 30 meV) the ab-plane, normal state, their optical conductivity is well described by a Drude model. In accord with the dc conductivity, the electronic scattering rate is linear in temperature, suggesting moderately strong coupling of charge carriers to some thermally generated excitation. At Tc,, infrared spectral weight moves into the zero-frequency delta function of the superconductor; the strength of this delta function gives the superfluid density. In a clean metallic superconductor the superfluid density is essentially equal to the conduction electron density. In contrast, in the high- Tc materials only about 25% of the ab-plane spectral weight joins the superfluid. The rest remains in finite-frequency, midinfrared absorption. In a one-component picture of the conductivity, this small fraction requires extremely strong coupling, ten times as large as what the dc conductivity gives.

Work at Florida is supported by the US Department of Energy..