# Sign of the wavenumber

1. Sep 5, 2008

### daudaudaudau

Hi.

In electromagnetics, a material(linear,isotropic,homogenous) with constitutive parameters $$\epsilon$$ and $$\mu$$ has the wavenumber $$k^2=\omega^2\epsilon\mu$$. Consequently $$k=\pm\omega\sqrt{\epsilon\mu}$$. Does this mean that $$\omega$$ 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.

2. Sep 6, 2008

### Antenna Guy

$$k=\frac{2\pi}{\lambda}$$

Negative k accounts for the opposite direction. As a wave approaches you, it is also moving away at the same rate (expanding spherically about the source).

Regards,

Bill