Quantum physics describes the free radiation field and the systems of particles (like atom, molecules and solids) as well as their interaction in terms of absorption, emission and diffusion of photons. These processes are at the basis of all experimental techniques for the study of material systems, their ground state properties and excitations using light as a probe. They can be described using perturbation expansions, at the various levels of approximations. Another important point considered in this introduction is the difference between single-particle and and many-electron description of the initial, intermediate, and final states
