  Very high energy (VHE) gamma-rays undergo pair production with low energy
photons of background radiation fields. This leads to an attenuation of the
primary gamma-ray flux of extragalactic sources in the interaction with the
extragalactic background light (EBL) which stretches from ultraviolet to
far-infrared wavelengths.
  In the presence of magnetic fields, gamma-rays could oscillate into
hypothetical axion-like particles (ALPs). This might lead to a reduced opacity
of the Universe for VHE gamma-rays, as ALPs circumvent pair production. Here,
the impact of photon-ALP conversions on the spectrum of PKS1424+240 is
demonstrated. A lower limit on the redshift of this blazar was recently
determined to be $z \geqslant 0.6035$, making it the farthest source ever
observed at VHE energies. Under the assumption of a specific magnetic field
scenario and EBL model, photon-ALP couplings are derived that lead to an
overall concave intrinsic blazar spectrum.
