  The reflection of a normally incident wideband pulse by a half-space whose
permittivity and permeability obey the one-resonance Lorentz model is
calculated. The results are compared with those from frequency-domain
reflection analysis. In the spectral regime wherein the real parts of the
permittivity and the permeability are negative, as well as in small adjacent
neighborhoods of that regime, the time-domain analysis validates the conclusion
that it is sufficient to ensure that the imaginary part of the refractive index
is positive - regardless of the sign of the real part of the refractive index.
