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Superconductors are really good at conducting electricity. Should they not reflect light very well too?

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    $\begingroup$ Super inductors are really good at conducting DC electricity. $\endgroup$
    – Johannes
    Jul 22, 2013 at 17:41
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    $\begingroup$ Welcome to SE.physics. Before asking a question, better to check if it has not been answered. This question is the exact analogue of this one: physics.stackexchange.com/q/2409/16689 where it is discussed that superconductors are really good mirror for wave-energy below the superconducting gap. $\endgroup$
    – FraSchelle
    Jul 22, 2013 at 19:05

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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.

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