  In this paper, we study the influence of the axion-plasmon, as proposed in
(Phys. Rev. Lett. 120, 181803 (2018)) on the optical properties of the
Schwarzschild black hole. Our aim is to provide a test to detect the effects of
a fixed axion background using black holes. To accomplish our goal, we explore
the effect of the axion-plasmon coupling on the motion of photons around the
Schwarzschild black hole and check the possibility of observing those effects
upon the black hole shadow, the gravitational deflection angle, Einstein rings
and shadow images obtained by radially infalling gas on a black hole within a
plasma medium. We find that these quantities are indeed affected by the
axion-plasmon coupling parameters which consequently generalize some of the
well-known results in the literature. It is shown that the size of the black
hole shadow decreases with increasing axion-plasmon if observed from
sufficiently large distance.
