  We study the photon's motion around a black hole in the presence of a plasma
whose density is a function of the radius coordinate by a renewed ray-tracing
algorithm and investigate the influence of the plasma on the shadow of the
black hole. The presence of plasma affects not only the size but also the shape
of black hole shadow. Furthermore, the influence of plasma on trajectories of
photons depends on the frequency of photon. For the high-frequency photons, the
influence is negligible, on the contrary, the trajectories of low-frequency
photons is affected significantly by the plasma. Interestingly, it is also
found that the black hole image would take on a multi-ring structure due to the
presence of plasma.
