The response of a metal to electromagnetic radiation is determined by the frequency-dependent conductivity. This in turn depends on the available mechanisms for energy absorption by the conduction electrons at the given frequency. The electronic excitation spectrum in the superconducting state is characterized by an energy gap 2Eg. So we expect the AC conductivity to differ substantially from its normal state at frequencies small compared with 2Eg/h, and to be essentially the same in the superconducting and normal states at frequencies large compared with 2Eg/h. The value of 2Eg/h is typically in the range between microwave and infrared frequencies. In the superconducting state, an AC behavior is observed that is indistinguishable from that in the normal state at optical frequencies. Deviations from the normal state behavior first appear in the infrared. At microwave frequencies AC behavior fully displaying the lack of electronic absorption characteristic of an energy gap becomes completely developed.
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