# Is this definition of complex wave number in dispersive media correct?

In Griffith's Introduction to Electrodynamics (4th edition, p.421), the complex wave number in the section on dispersive media is defined as $$\tilde{k}=\sqrt{\tilde{\epsilon}\mu_0}\omega$$. Why is the vacuum permeability used? These are electromagnetic waves in matter, right?

I have checked the errata, but have not found a comment on this nor on any assumptions made earlier in the text.

• On page 420 Griffith states, “For simplicity, I assume the magnetic force is negligible”. – Farcher Feb 15 at 21:44

Usually the dispersion comes solely from the $$\epsilon$$. In most materials (and certainly most that are transparent) the relative permeability is close to unity. I imagine Griffith is just ignoring the possibility of magnetic effects.
Magnetic effects are ignored and the relation $$\epsilon_0 \mu_0 = 1/c^2$$ was used.