The transfer of heat through empty space is known as radiation. There is no medium needed in this form of heat transfer; radiation works even in and through a perfect vacuum. For instance, energy from the sun travels through the vacuum of space before the transfer of heat warms the earth.
Energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules. This includes electromagnetic radiation such as radio waves, visible light, and x-rays, particle radiation such as α, β, and neutron radiation and acoustic radiation such as ultrasound
sound, and seismic waves. The transfer of heat by radiation does not require the presence of an intervening medium.
Heat transfer by radiation is fastest (at the speed of light) and it suffers no attenuation
in a vacuum. This is how the energy of the sun reaches the earth.
In heat transfer studies we are interested in thermal radiation, which is the form of
radiation emitted by bodies because of their temperature.
All bodies at a temperature above absolute zero emit thermal radiation. Radiation is a volumetric phenomenon, and all solids, liquids, and gases emit, absorb, or transmit radiation to varying degrees. However, frequently, it is more convenient to treat it as a surface phenomenon.
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