ABSTRACT
T his chapter is about the interaction of a two-state system with a sinusoidally oscillating field (either electric or magnetic), whose oscillation
frequency is nearly resonant with the natural frequency of the system, i.e. the difference in energies between the two states. The oscillating field will be treated classically, and the linewidth of both states will be taken as zero until near the end of the chapter, where relaxation will be treated phenomenologically. The organization of the material is historical because this happens to be a logical order of presentation-magnetic resonance of a classical moment is discussed before taking up a quantized spin; the density matrix is introduced last and used to treat systems with damping, which is a useful prelude to the application of resonance ideas at optical frequencies, and to the many real systems which have damping.
