In the Jaynes-Cummings model, a two-level atom couples to a single cavity mode with strength g. The vacuum Rabi splitting 2g lifts the dressed-state degeneracy — even with zero photons, the atom-cavity coupling shifts the energy levels. The resulting Rabi oscillations between |e,0⟩ and |g,1⟩ are a hallmark of strong coupling cavity QED, visible in both time evolution and transmission spectra.