Figure 15: Frequency dependence of the real and imaginary parts of the conductivity, and , in niobium at various temperatures above and below the critical temperature  K. The transmission through a 150 Å thick film on sapphire was measured by a Mach-Zehnder interferometer; the stars were obtained by reflection measurements (after [97]). The lines are calculated using the theory of Mattis and Bardeen. The inset shows the temperature dependence of the penetration depth evaluated from the at the lowest frequency.