RESONANCE

When a glass is struck, it will vibrate at its natural frequency determined by its shape and material. Conversely, when it is "excited" by the impinging sound of that frequency, it will begin to vibrate. There are singers whose voice is claimed to be so strong it can break a glass, but I suspect what does the trick is the quality of the glass and the knowledge which note to sing as much as the strength of their voice.

The phenomenon of making something vibrate strongly by exciting it at its natural frequency is known as resonance. There are many more examples in acoustics and mechanics: the corresponding piano string will begin to vibrate if one sings a certain note (step on the pedal to free all strings for vibration). A loose panel in a bus will begin to vibrate audibly when the engine reaches a certain number of revolutions per minute, and in bygone times, when bridges were less solid and armies less motorized, troops marching across a bridge were always given a "break step" command, for it had happened that the rhythm of their march coincided with the natural frequency of the bridge, and its resonant vibrations, fed by the continuing march, became so large that the bridge collapsed.

Resonance can also be electrical: it is used to tune in only one of the signals received in the antenna by hundreds of transmitting radio or TV stations, which differ from each other in the frequency of the alternating currents they induce in the antenna. (They sure don't differ much in their selection and omission of the news.)

But the resonance we will be concerned with below is one that occurs in the nuclei of the atoms of certain chemical elements. When placed in a strong magnetic field and tickled by an electric field with the right frequency (in the radio frequency band, i.e., lower than infrared and visible fight), certain nuclei will resonate: Like the piano string under the singer's voice, they absorb the incoming energy. When you stop singing, you can hear the excited piano string vibrate, and when you stop tickling with the radio frequency, you can observe the nucleus radiating it.