Micro view of NMR

NMR is an instrumental technique that uses photons of wradio frequency energy to cause a transition, or change in state, in an atom. Radio is used because transitions at the atomic level are quantized, and the amount of energy needed to cause these transitions happens to fall in the radio region of the electromagnetic spectrum. There has to be the right amount of energy to cause a change of state, too much or too little and no change occurs.

There are many transitions in atoms. The ones of interest in NMR are quantum spin transitions.