  We discuss spectral distortions, time delays and refraction of light in an
axion or axion-plasma background. This involves solving the full set of
geodesic equations associated to the system of Hamiltonian optics, allowing us
to self-consistently take into account the evolution of the momentum, frequency
and position of photons. We support our arguments with analytic approximations
and full numerical solutions. Remarkably, the introduction of a plasma enhances
the sensitivity to axion-induced birefringence, allowing these effects to occur
at linear order in the axion-photon coupling even when the axion background is
not present at either the emission or detection points. This suggests a general
enhancement of axion-induced birefringence when the background refractive index
is different from one.
