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In electromagnetics, a material(linear,isotropic,homogenous) with constitutive parameters [tex]\epsilon[/tex] and [tex]\mu[/tex] has the wavenumber [tex]k^2=\omega^2\epsilon\mu[/tex]. Consequently [tex]k=\pm\omega\sqrt{\epsilon\mu}[/tex]. Does this mean that [tex]\omega[/tex] can actually be negative, and if so, when is it the case? It seems strange to me, but some guy told me today that a negative wavenumber was indeed possible.

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# Sign of the wavenumber

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