Radiation is the most curious form of heat transfer. It does not require any medium for its propagation. This tutorial explains how thermal radiations are generating, and how it is propagated through space with the help of Quantum theory and Wave theory.


We learned that heat radiations are electromagnetic radiations and do not require any medium for its propagation. More precisely the radiation heat transfer is more effective in vacuum than any other medium. See modes of heat transfer


In order to understand, how radiation heat transfer occurs, we have the help from quantum theory. According to this, when a solid body is heated, the atoms and molecules are gets energized and they rose to higher excited states by absorbing this energy. When these atoms and molecules returns to their normal or lower energy ground states they will emit the absorbed energy as electromagnetic radiations. Since this process is spontaneous in any body having temperature greater than zero Kelvin, therefore all bodies emit some form of radiations. This emitted energy is not continues but in the form of packets of energy called as 'quanta' after Max Planck's quantum theory. According to max Planck's quantum theory value of energy emitted,
E= hf
Where h is Planck's constant and f is frequency radiation emitted.

Propagation of heat transfer as radiation can also understand on the basis of wave theory. According to this theory all space in universe is filled with a hypothetical medium known as ether, electromagnetic radiations are propagated through this ether. When a molecule emits energy due to its vibration its stored energy gets reduced resulting in decreasing of its internal energy. This emitted energy travel through space until strike on another body which may absorb, reflect or transmit this energy depends upon striking material, wave length of radiation, surface finish etc.

Large amount of energy from the sun reaches earth as heat and light radiations. Major heat carrier radiation in electromagnetic series is infrared radiations. To learn more about heat radiations and governing laws see radiation laws