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Crystal goblets can sing

Q: What causes crystal water goblets to "sing" when you run a wet fingertip around their rim? --Theresa, Orange, California

A: ...a glass of water may be made to emit a tone merely by the friction of the finger-tip upon the rim of the glass; for in this water is produced a series of regular waves. --Galileo Galilei, 1638.

[Corel] Ancient Venetian wine glass

A: Galileo came up with the answer 364 years ago.

When I rub my wet, clean fingertip around the rim of a goblet, my finger catches and releases on tiny imperfections of the goblet's rim. When my finger catches, it pulls the glass molecules apart until the tension becomes too great. Then the finger slips. As my finger travels around the rim, the disturbances create a train of impulses that vibrate the glass molecules of the goblet. The glass sings when my finger causes the goblet to vibrate at its natural frequency--the frequency it can best respond to. The resonant vibrations disturb the surrounding air and create audible pressure waves. The waves travel through the air to my ears and I hear the song.

Hit or disturb any object made of an elastic material and it will vibrate at its own special set of natural frequencies: the clack of a dropped pencil hitting the floor, the rich tones of a struck gong. The object's properties, like its shape and elasticity, determine its natural frequencies for resonance.

Note to experimenters: You need squeaky-clean fingers to make the goblet sing.

(Answered July 26, 2002)

Further Surfing:

Tom Henderson's The Physics Classroom

William Wilde Zeitler: The glass armonica