The Ultrasonic Grating Apparatus is both a unique demonstration of the interaction of longitudinal sound waves and transverse light waves and also a method for the precise determination of the speed of sound in liquids, which is otherwise difficult to do in the undergraduate lab. The core of the apparatus is a parallel-sided glass tank that houses the liquid sample and contains a lead zirconate titanate (PZT) ultrasonic transducer. The transducer is energized by an RF signal in the 8-12 MHz range. Resonances can be found to set up a standing wave field in the liquid, which results in a periodic variation of the liquid's refractive index along the tank.