  We observed optical bistability as well as oscillations in the upper bistable
branch when cold ytterbium atoms are dispersively coupled to a high finesse
optical cavity. Comparable previous observations were explained by atomic
density oscillations in case of a Bose-Einstein-Condensate or optical pumping
in case of a cold cloud of cesium. Both explanations do not apply in our case
of thermal atoms with only a single ground state. We propose a simple two-level
model including saturation to describe our experimental results. This paper
introduces the experimental setup, derives the mentioned model and compares it
to our experimental data. Good quantitative agreement supports our model.
