  Materials that exhibit loss or gain have a complex valued refractive index
$n$. Nevertheless, when considering the propagation of optical pulses, using a
complex $n$ is generally inconvenient -- hence the standard choice of
real-valued refractive index, i.e. $n_s = \RealPart (\sqrt{n^2})$. However, an
analysis of pulse propagation based on the second order wave equation shows
that use of $n_s$ results in a wave vector \emph{different} to that actually
exhibited by the propagating pulse. In contrast, an alternative definition $n_c
= \sqrt{\RealPart (n^2)}$, always correctly provides the wave vector of the
pulse. Although for small loss the difference between the two is negligible, in
other cases it is significant; it follows that phase and group velocities are
also altered. This result has implications for the description of pulse
propagation in near resonant situations, such as those typical of metamaterials
with negative (or otherwise exotic) refractive indices.
