All objects emit heat radiation

Spread of heat

The thermal radiation¶

Hot bodies always emit thermal radiation. Through this radiation, heat can be transferred without the involvement of matter.

Thermal radiation can penetrate air and can also be seen behind window panes. Most bodies, however, absorb thermal radiation and are heated by it. In general, incident thermal radiation can be absorbed, reflected or transmitted by a body; The sum of these three parts always results .


  • Bodies with a dark surface absorb a larger part of the radiation than bodies with a light surface; therefore they heat up faster. The rest of the radiation is reflected.
  • Shiny metal surfaces reflect a large part of the heat radiation, the rest of the heat radiation is absorbed.
  • Transparent glass only absorbs a small part of the heat radiation. The thermal radiation is partly reflected, and partly it can penetrate the glass. Likewise, stagnant waters are only warmed by the rays of the sun near the surface; Thermal radiation cannot penetrate thick layers of water.

In quantitative terms, the heat radiation emitted by a hot object can - similar to the other types of heat transport - through the heat flow to be discribed:

This quantity is sometimes also called "irradiance", its unit is watt per square meter. For example, in Central Europe the sun can have an irradiance of around on a cloudless summer day achieve. [4]

Any object that can absorb thermal radiation also emits it efficiently. An ideal "black body", as it is often assumed as a simplifying model in thermodynamics, can on the one hand reduce the thermal radiation that hits it absorb and on the other hand emit the best possible heat radiation corresponding to its temperature. The radiation law named after the discoverers Josef Stefan and Ludwig Boltzmann applies to such a body, according to which the intensity of the thermal radiation is proportional to the fourth power of the absolute temperature (measured in Kelvin):

The occurring constant is referred to as the “Stefan-Boltzmann constant”. The thermal radiation itself is - like light - electromagnetic waves. A hot body does not only emit a single wavelength, but rather a continuous spectrum of different wavelengths. The distribution of the radiated amounts of energy at a certain temperature is in turn dependent on the wavelength dependent.

Can an object not have thermal radiation absorb or emit, then in the above formula (7) on the right-hand side of the equation a number factor must also be added which takes into account the degree of emission or absorption of the object:

The degree of emission or absorption an object can be of different sizes at different wavelengths.

As the temperature rises, the spectrum of the radiation shifts to shorter wavelengths. While at low temperatures the emitted wavelengths are predominantly in the infrared range, a hot surface begins at a temperature of at least (around ) to glow visible to the human eye. This connection can be quantitatively described by the "Vienna law of displacement" named after its discoverer Wilhelm Wien:

Here describes the maximum of the spectral energy density at a certain (absolute) temperature . For a temperature of about the thermal radiation of a black body corresponds to a very good approximation with the radiation spectrum of the sun.