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thermal radiation

In a narrow sense: synonym for infrared radiation.

In a more general sense: The electromagnetic radiation emitted by every body with non-zero temperature due to the laws of thermodynamics. The properties of this radiation (in particular: its spectrum) depend on the body’s temperature – in the simplest case, that of what is called a blackbody, they even depend on nothing but the temperature and some universal constants.

For everyday temperatures, such as those of a hotplate, thermal radiation is emitted mainly in the form of infrared radiation. At higher temperatures, significant amounts of visible light are emitted, as well: a hotplate that is very hot indeed looks dark red or even light red; molten metal looks yellow or even white. In more extreme situations, thermal radiation can have energies that are even higher – for instance, the gases in the accretion discs of black holes are so hot that they emit great amounts of thermal radiation in the x-ray region.

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thermal radiation

In a narrow sense: synonym for infrared radiation. In a more general sense: The electromagnetic radiation emitted by every body with non-zero temperature due to the laws of thermodynamics. The properties of this radiation (in particular: its spectrum) depend on the body's temperature - in the simplest case, that of what is called a blackbody, they even depend on nothing but the temperature and some universal constants. For everyday temperatures, such as those of a hotplate, thermal radiation is emitted mainly in the form of infrared radiation. At higher temperatures, significant amounts of visible light are emitted, as well: a hotplate that is very hot indeed looks dark red or even light red; molten metal looks yellow or even white. In more extreme situations, thermal radiation can have energies that are even higher - for instance, the gases in the accretion discs of black holes are so hot that they emit great amounts of thermal radiation in the x-ray region.

temperature

In systems consisting of many particles, be they solid bodies, fluids or gases, the constitutents are in constant, chaotic motion: the atoms in a solid crystal oscillate a bit, the molecules of a gas are in rapid, disordered motion, and so on. The average energy with which each constitutent contributes to every part of the disorderly motion is the same, and it is called the temperature of the system. High average energy corresponds to high temperature - atoms vibrating wildly, gas molecules zipping around very fast -, low average energy to low temperature. In a slightly different context, certain mixtures of electromagnetic radiation can be assigned a temperature ("radiation temperature"), a single parameter that completely defines the basic properties of the radiation (more precisely, its spectrum). It corresponds to the thermal radiation emitted by a hot body with precisely that temperature. In physics, temperature is measured in Kelvin, in everyday life, depending on the country, in Fahrenheit or Celsius.

radiation

In a general sense: Collective name for all phenomena in which energy is transported through space in the form of waves or particles. In a more restricted sense, the word is often used synonymously with electromagnetic radiation.

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thermal radiation

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