Raman scattering is a powerful tool for revealing the vibrations of molecules, but as a nonlinear optical phenomenon, its signals can change via mechanisms like resonance enhancement that have no direct analogue in infrared spectroscopy. In this work, complementary measurements conducted on 4-nitrophenol and its conjugate base allow students to directly control the variables responsible for enhancement. Changing sample UV-vis absorbance with pH and excitation color with the addition of a diode laser allows students to explore the criteria for resonance Raman scattering: only the conjugate base and 405 nm laser in combination produce enhancement. Adjustments to the experiment make it suitable for either an upper-level undergraduate course labor an independent inquiry project.