Hi, I'm hoping to acheive some intuition regarding the real and imaginary parts of the complex conductivity. The rule of thumb seems to be that the real part is due to the absorptive/dissipative processes and the imaginary part absorbs no work (over a time average). However, I feel like this doesn't quite explain everything. QUESTION 1. For instance, what exactly is absorption? At low frequencies (compared with the scattering rate) the real part of the conductivity dominates in the Drude model. In this case the light is mostly reflected (this is why metals are shiny). However, in a superconductor or perfect conductor, the light is completely reflected. I've seen a proportionality of the absorption coefficient to the real part of the conductivity. QUESTION 2: Is this proportionality always true (σ1 ~ alpha), and if so how can one interpret it? There is also the conservation of energy equation R + T + A = 1. This is why I'm having trouble with the distinction between absorption and reflection. It seems that in the metal, the light is absorbed (just as it would be for an interband transition), the electron is promoted to a higher energy state i.e. it is oscillated, and then the oscillating electron produces radiation. QUESTION 3: But if this (reflection from a metal) is a real part of the conductivity process, then how can I understand a superconductor, which is a perfect reflector but has essentially entirely an imaginary part of the conductivity? It also temporarily "absorbs" light (via charge carrier oscillation) before emitting it back towards the source. QUESTION 4: Is absorption mainly due to Joule heating and interband transitions, which by integrating F dx over a current oscillation can be shown to be due to σ1? If anyone can address these conceptual issues I have and/or could provide intuitive ways to think about the complex conductivity (especially in terms of solid state physics) I would be REALLY grateful!