Spectroscopy pertains to the dispersion of an object’s light into its component colors (i.e. energies). Spectral lines are the result of interaction between a quantum system (usually atoms, but sometimes molecules or atomic nuclei) and a single photon. The direction and polarization of the new photons will, in general, correlate with those of the original photon. Practitioners, researchers and students will find this book useful in their daily working activity, in teaching and in studies.