Hi I have two books about electrodynamics that solve Maxwell's Equations. The first one uses the assumptions 1) Linear regime (i.e. not strong fields) 2) Isotropic medium (so disregard tensor nature of ε) 3) Transparent medium (i.e. a real ε) 4) No dispersion of ε In the second book, they use (1)-(3) as well, but (4) is now stated as 4) ε is time-invariant Now, my questions is: How can time-invariance of ε be the same as ε not having dispersion? Because if ε is constant in time, then Fourier-transforming it will give me a delta-function. So ε *will* depend on ω. What is wrong with my reasoning so far? Thanks for any help. Best, Niles.