  Data students collect from the typical advanced undergraduate laboratory on
Saturated Absorption Spectroscopy (SAS) of rubidium can be used to measure the
isotope shift and thus leads to an estimate of the isotopic ground state energy
shift. This helps students refine their `picture' of the atomic ground state.
We describe theoretically why this laboratory works well with free-running
laser diodes, demonstrate it experimentally using these lasers tuned to either
principal near-infrared transitions, and show an extension of the laboratory
using the modulation transfer spectroscopy method.
