Thermal radiation is radiation that things make because they are warm. It may be felt as heat or seen as light. It is a form of heat transfer that is moved from one place to another by electromagnetic radiation waves or rays. It does not require a form of matter to be transferred.
All matter generates thermal radiation. Warmer things make more radiation, and the electromagnetic waves are shorter. Most things on Earth are warm enough to make infrared radiation. Incandescent light bulbs are so hot, their radiation includes visible light.
- Robert Siegel, Thermal Radiation Heat Transfer, Fourth Edition (London; New York: Taylor & Francis, 2002), p. 1
- Mahesh M. Rathore; Raul Raymond Kapuno, Engineering Heat Transfer (Sudbury, MA: Jones & Bartlett Learning, 2011), p. 807