An advanced undergraduate laboratory experiment which studies the response of a bar to forced transverse vibrations is described. The amplitude and phase of vibration are monitored using a relatively simple optical system involving Moire fringes. Light is passed through two Ronchi rulings onto a photocell, the photocell output being a direct measure of the phase and the amplitude of vibration. The photocell output and the oscillator voltage used to drive the bar can be applied to the vertical and horizontal input, respectively, of an oscilloscope to produce Lissajous figures. These figures can then be studied and related to the properties of forced vibration. Typical results for resonance curves with and without external damping are given.