6C QUANTUM-MECHANICAL PROPERTIES OF RADIATION
When electromagnetic radiation is emitted or absorbed, a permanent transfer of energy from the emitting object or to the absorbing medium occurs. In order to describe these phenomena, it is necessary to treat electromagnetic radiation not as a collection of waves but rather as a stream of discrete particles called photons or quanta. The need for a particle model for radiation became apparent as a consequence of the discovery of the photoelectric effect in the late nineteenth century.