  The electric permittivity and magnetic permeability for the quark-gluon
plasma (QGP) is calculated within the hard-thermal-loop (HTL) perturbation
theory. The refractive indices in the magnetizable and non-magnetizable plasmas
are calculated. In a magnetizable plasma, there is a frequency pole
$\omega_{mp}$ in the magnetic permeability and the refractive index. The
refractive index becomes negative in the range $\omega\in[k,\omega_{mp}]$,
where $k$ is the wave number, but no propagating modes are found. In a
non-magnetizable plasma, the magnetic permeability and the refractive index are
always positive. This marks the main distinction of a weakly coupled plasma
from a strongly coupled one, where the negative refraction is shown to exist in
a holographic theory.
