Did we cover this in the lectures? I don't really remember.
It is in Ashcroft and Mermin and also in some notes of mine from the 3rd yr SS course I did at Griffith.
The Drude model explains how visible and other low frequency light are reflected off the surface of metals yet to light above a certain frequency, metals become very transparent.
This is a consequence of the drude model's AC conductivity in metals when an external electric field is applied which is dependent on the frequency of electric field.
When the frequency of the electric field is much greater than the relaxation time^-1, the real part of the conductivity (largely) disappears and the solutions for k in the wave equation become real. This allows the high frequency (usually ultraviolet) light to propagate through metals.
For lower frequencies, the solutions for k in the wave equation are imaginary resulting in an exponential decay of the electric field as it travels into the metal.
The transition frequency between these two cases is known as the plasma frequency.
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