Thermal radiation is the energy emitted by bodies because of their temperature level. Other types of radiation include gamma rays, xrays, for instance. Radiation is often treated as electromagnetic waves that propagate according to Maxwell's classic electromagnetic theory. Radiation may also be treated as photons as prescribed in Max Planck's concept of the quantum of energy. The electromagnetic theory has been used to predict the radiant properties of materials, while the quantum theory has been used to predict the amount of radiant energy emitted by a body because of its level of temperature.