Interaction of Radiation with Matter The Classical Picture

It is perhaps worthwhile to mention that the velocity (v) of electromagnetic radiation decreases as it passes through a condensed medium (e.g., a liquid or solution). The ratio of its speed in a vacuum (c) to its velocity in the medium is called the index of refraction (n) of the medium  

3 INTERACTION OF RADIATION WITH MATTER THE CLASSICAL PICTURE 

Radiation is scattered when the direction of propagation of the photon is shifted by some angle, the result of passing 

In the case of NMR spectroscopy we will be concerned only with absorption and emission of rf radiation. Quantum mechanics, the field of physics that deals with energy at the microscopic (atomic) level, allows us to define selection rules that describe the probability for a photon to be absorbed or emitted under a given set of circumstances. But even classical (i.e., pre-quantum-mechanical) physics tells us there is one requirement shared by all forms of absorption and emission spectroscopy For a particle to absorb (or emit) a photon, the particle itself must first be in some sort of uniform periodic motion with a characteristic fixed frequency. Most important, the frequency of that motion must exactly match the frequency of the absorbed (or emitted) photon  

All of the above forms of microscopic motion are what we might describe as intrinsic. That is, the motion takes place all by itself, without intervention by any external agent. However, it is possible under certain circumstances to induce particles to engage in additional forms of periodic motion. Still, to achieve resonance, we need to match the frequency of this induced motion with that of the incident radiation [Eq. (1.5)]. 

---