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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.

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

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Magnetic effects are ignored and the relation $\epsilon_0 \mu_0 = 1/c^2$ was used.

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