Which phenomenon describes energy exchange between spin states at a specific frequency?

• A. Precession
• B. Resonance
• C. Relaxation
• D. Dephasing

Answer: (B)

Resonance describes the situation where energy exchange between spin states occurs at a specific frequency. In a magnetic field, a spin-½ system has two energy levels separated by the energy gap corresponding to the Larmor frequency. When an RF field is applied at that exact frequency, it can efficiently drive transitions between the levels, exchanging energy with the spins. This selective, frequency-specific energy transfer is what resonance captures. Precession refers to the rotation of the magnetization around the field and doesn’t by itself denote energy transfer between levels; relaxation is about returning to equilibrium with energy loss, and dephasing is the loss of phase coherence.