We describe an undergraduate plasma physics experiment using an easily constructed and inexpensive apparatus that nevertheless involves the more advanced physics of the interaction between plasma and electromagnetic radiation. The experiment is based on the readily available plasma of a fluorescent lamp. The lamp is placed inside a resonant cavity and the shifts in the frequencies of cavity resonances are used to determine the plasma density as a function of time. We describe a circuit that allows measurements to be made of the decay of the afterglow plasma, and show experimentally determined decay rates and densities that are consistent with the literature on fluorescent lamps and gas discharges.