Conversely, heat transfer by thermal radiation requires a temperature gradient but not parcipating medium. All bodies at a temperature above absolute zero emit thermal radiation. Consider for example a hot object in an evacuated container whose walls are cold. Conduction and convection are not possible, so heat is transferred strictly by radiation.
Heat is transferred from the Sun to the Earth via radiation.
However, the actual mechanism of radiation transfer is not yet fully understood. Maxwell's electromagnetic theory and Planck's quantum theory have both been used toward this goal. This portion of the spectrum is therefore usually referred to as the thermal radiation range. There are two important considerations to be mentioned at this point. The magnitude and distribution vary with the type of material and its temperature.