The fundamental physical and chemical processes responsible for luminescence will be introduced and explained in this chapter. The classical division of luminescence into Fluorescence and Phosphorescence will be presented along the similarities and differences between these two processes. Starting with a simple introduction to light and electromagnetic waves and moving to atomic and molecular systems, the text will allow the reader to intuitively understand why physical systems absorb and emit light. This chapter represents the theoretical foundations on which the rest of the book is built upon, the platform from which all other chapters and processes explained therein can be adequately understood and apprehended.
Keywords: Atom, Electromagnetic wave, Electronic excitation, Excited state, Fluorescence, Grotrian diagram, Jablonski diagram, Light, Luminescence, Molecule, Non-radiative de-excitation, Phosphorescence, Photochemistry, Photon, Potential energy curve, Radiative de-excitation, Spin.