Why aren't superconductors shiny? Superconductors are really good at conducting electricity. Should they not reflect light very well too?
 A: A photon can only excite a pair in the superconductor if the photon's frequency is higher than the energy gap, so above this frequency the superconductor won't be perfectly reflective, and behaves somewhat like a dielectric with regard to optical properties. The analogy is not perfect since the dispersion of the pairs is not linear, and there may be excitations involved other than photons and pairs.
But, if the photon's energy is lower than this, then the photon can't excite pairs inside the superconductor, and has to produce a reflected photon.
For tin far below the critical temperature, this frequency is roughly 2.7 $\times 10^{11}$ Hz according to BCS theory, which corresponds to a wavelength of about a millimeter, much longer than the wavelength of visible light.
So for radio waves, a large enough slab of superconducting tin will be an excellent mirror, but looks basically like any other material at visible wavelengths and above.
For more dramatic examples of superconductors that look like mundane dielectric materials, magnesium diboride and certain perovskites spring to mind.
