Quantum Properties of Matter

In this chapter we analyze empirical phenomena which are explained by taking into account the quantum nature of matter. We investigate the peculiar behavior shown by the heat capacity in solids at low temperature, introducing the concept of vibrational phonons. Following the approach of Einstein and Debye, the solid can be seen as a gas of phonons characterized by a specific quantum thermal distribution, similar to the one of the photons of the black-body radiation. We also consider the quantization of the energy levels proposed in 1913 by Bohr for the hydrogen atom and the three main mechanisms of electromagnetic transitions for atoms and molecules: absorption, spontaneous emission, and stimulated emission.