Resonance on the nuclear scale is somewhat analogous to mechanical resonance. It is a response of an object or system that vibrates in step or phase.
A glass can be broken by the voice of a singer or a bridge can oscillate and collapse when the marching rhythm of a column of soldiers or the undulation caused by strong winds correspond to its own structural resonance frequency, such as the Tacoma Narrows Bridge across the Pudget Sound (Figure 02-08).
This correspondence of frequencies allows energy to be transferred from the external world (the soldiers' legs) to a given physical system (the bridge).
Similarly, a resonance phenomenon will occur when an electromagnetic wave of appropriate frequency (equal to the Larmor frequency) reaches the nuclei; then, nuclei located in the state of lower energy will be transferred to the state of higher energy.