The energy difference between two nuclear spin states in an external magnetic field B is given by [itex]\Delta E = \gamma \hbar B[/itex] Why does resonant absorption of electromagnetic radiation by nuclear spins take place at the Larmor frequency? Shouldn't it be that the photon energy is equal to the energy difference between the two nuclear spin states? Or in this case, does the radiation freuquency just happen to be equal to the Larmor frequency? What is the value of invoking the Larmor frequency into the explanation of NMR? Wouldn't it just be simpler to say the photon energy must be equal to the energy difference between spin states?