Cherenkov light rings

Let’s suppose a muon emits Cherenkov light while travelling in a medium along a straight line. Let’s suppose the motion is perpendicular to a wall which is instrumented with photomultipliers.

Question: Should these PMTs "see" a ring or a solid circle?

Note: The true answer is a ring. Indeed I know that for example in SuperKamiokande muons produce ring shaped "pictures". But in my intuition they should produce solid circles, as the following drawing could explain.