Using an ALTA II reflectance spectrometer, the USGS digital spectral library, graphs of planetary spectra, and a few mineral hand samples, one can teach how light can be used to study planets and moons. The author created the hands-on, inquiry-based activity for an undergraduate planetary science course consisting of freshman to senior level science majors and nonmajors. The activity follows a guided-inquiry approach that unites lab-based investigation of minerals with online datasets, and enables students to import, configure, graph, analyze, and compare data using spreadsheet software. The activity includes two parts: (1) analysis of mineral hand samples and spectra, and (2) exploration of planetary reflectance spectra. The activity culminates with students using techniques analogous to those employed by planetary scientists to establish rationales for determining compositional information about, and categorizing and identifying, several planetary bodies in the solar system based solely on reflectance spectra.