Lasers, LEDs and photodetectors all share the same basic physical principles. The rate equations provide a fundamental description of optical emission and absorption. However, as in Chapter 2, the gain and spontaneous recombination terms often appear as phenomenological terms. Maxwell’s equations and the Poynting vector explain emission, absorption and transport in terms of the classical theory of the microscopic dipoles. While quite successful, the description does not account for the quantum nature of matter and light, and does not explain basic phenomena such as the spontaneous emission.