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. Any matter which is at any temperature above absolute zero (0 K or -273°C) gives out and absorbs thermal radiation. Like light, thermal radiation travels through vacuum at a speed of approximately 3X10^8 metres per second. Thermal radiation can also travel in all directions. Dark, dull surfaces give out more thermal radiation while bright and shiny surfaces give out comparatively less thermal radiation. Those surfaces which give out thermal radiation well also absorb thermal radiation well.
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