  Light propagation through a normal medium is determined not only by the real
part of the refractive index but also by its imaginary part, which represents
optical gain and loss. Therefore, two media with different gain and loss
landscapes can have very different transmission and reflection spectra, even
when their real parts of the refractive index are identical. Here we show that
while this observation is true for an arbitrary one-dimensional medium with
refractive index $n(x)$ and its time-reversed partner with refractive index
$n^*(x)$, there exists a universal scaling that gives identical transmittance
and reflectance in these corresponding systems. Interestingly, these scaled
transmittance and reflectance reduce to their standard, unscaled forms in a
time-reversal invariant system, i.e., one without gain or loss.
