We analytically calculate the influence of a plasma on the shadow of a blackhole (or of another compact object). We restrict to spherically symmetric andstatic situations, where the shadow is circular. The plasma is assumed to benon-magnetized and pressure-less. We derive the general formulas for aspherically symmetric plasma density on an unspecified spherically symmetricand static spacetime. The formalism applies not only to black holes but also,e.g., to wormholes. As examples for the underlying spacetime model, we considerthe Schwarzschild spacetime and the Ellis wormhole. In particular, we treat thecase that the plasma is in radial free fall from infinity onto a Schwarzschildblack hole. The perspectives of actually observing the influence of a plasma onthe shadows of supermassive black holes are discussed.
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