  We study the propagation of light in the presence of a parity-violating
coupling between photons and axion-like particles (ALPs). Naively, this
interaction could lead to a split of light rays into two separate beams of
different polarization chirality and with different refraction angles. However,
by using the eikonal method we explicitly show that this is not the case and
that ALP clumps do not produce any spatial birefringence. This happens due to
non-trivial variations of the photon's frequency and wavevector, which absorb
time-derivatives and gradients of the ALP field. We argue that these variations
represent a new way to probe the ALP-photon couping with precision frequency
measurements.
