Introduction

Electromagnetic interactions play a central role in low-energy physics, chemistry, and biology. They are responsible for the cohesion of atoms and molecules and are at the origin of the emission and absorption of light by such systems. They can be described in terms of absorptions and emissions of photons by charged particles or by systems of charged particles like atoms and molecules. Photons are the energy quanta associated with a light beam. Since the discoveries of Planck and Einstein at the beginning of the last century, we know that a plane light wave with frequency ν, propagating along a direction defined by the unit vector u, can also be considered as a beam of photons with energy E = hν and linear momentum p = (hν/c)u. We shall see later on that these photons also have an angular momentum along u depending on the polarization of the associated light wave.

Conservation laws are very useful for understanding the consequences of atom–photon interactions. They express that the total energy, the total linear momentum, and the total angular momentum are conserved when the atom emits or absorbs a photon. Consider for example the conservation of the total energy. Quantum mechanics tells us that the energy of an atom cannot take any value. It is quantized, the possible values of the energy forming a discrete set Ea, Eb, Ec, . . . In an emission process, the atom goes from an upper energy level Eb to a lower one Ea and emits a photon with energy hν. Conservation of the total energy requires The energy lost by the atom on going from Eb to Ea is carried away by the photon.