Thermal Radiation

The radiant heat transfer includes the transfer of heat by electromagnetic radiation which occurs due to the temperature of a body. Most of the energy of this kind is in the infra-red area of the electromagnetic spectrum though some of it is in the visible area. The term thermal radiation is often used to differentiate this form of electromagnetic radiation from the other forms, like x-rays, radio waves, or gamma rays. The transfer of heat from a fire-place across a room in the line of sight is an illustration of the radiant heat transfer.

The radiant heat transfer does not require a medium, like air or metal, to occur. Any material which has a temperature above absolute zero gives off few radiant energy. Whenever a cloud covers the sun, both its heat and light reduce. This is one of the most well-known illustrations of heat transfer by thermal radiation.