  Considering a Schwarzschild black hole surrounded by a fully ionized hydrogen
plasma, we study the refractive effect and the pure gravitational effect of the
plasma on the shadow. The effects are treated in a unified formalism but
characterized by two independent parameters. For semi-realistic values of
parameters, we find their corrections to the shadow radius are both negligible,
and the gravitational correction can overtake the refractive correction for
active galactic nuclei of masses larger than $10^9M_{\odot}$. Since the
refractive effect is induced by the electromagnetic interaction, this result is
in sharp contrast to the textbook knowledge that the ratio of the gravitational
force to the electromagnetic force is $Gm_em_p/e^2=4.4\times10^{-40}$ in a
hydrogen atom. With unrealistically large values of parameters, we illustrate
the two effects on the light trajectories and the intensity map.
