Wine glasses and ringing bells.

Any dinner guest who has playfully rubbed a moist finger along the rim of a wine glass to make it "sing" has probably experienced the ethereal sound of vibrating glass. Such sounds also emanate from musical instruments known as glass harmonicas, which generally consist of arrays of wine glasses tuned to produce different notes. Much less familiar are the details of how these glasses vibrate to produce their tones.

To find out, physicist Thomas D. Rossing of Northern Illinois University in DeKalb brought a number of brandy snifters and wine glasses into the laboratory. Using a technique known as holographic interferometry, he produced images, which resemble contour maps, indicating the amount of motion at each point on a given glass. "It sings like a bell," Rossing concludes. As it vibrates, the glass rim repeatedly changes its shape from a circle to an ellipse, displaying a mode of vibration similar to that of a large church bell.