Thermal Radiation

Abstract

Among the various possibilities for excitation of molecules and atoms, thermal excitation has played a special role since ancient times. Therefore, thermally-excited radiation (or “temperature radiation”) has been extensively investigated. The crowning achievement of this work was the formulation and derivation of Planck’s thermal radiation law in 1900, and with it the discovery of the physical constant h, Planck’s constant or Planck’s quantum of action.

The fundamental experimental results can be summarized briefly:

1. All objects radiate energy towards each other. Warmer objects are thus cooled, while cooler objects are warmed. In order to demonstrate this transfer of heat via radiation, we must first avoid thermal conductivity. It is expedient to use two concave mirrors facing each other at a distance of several meters. At the focal point of one mirror, we place a radiometer ( a radiation thermopile). At the focus of the other, we can first hold a warm finger, then a beaker filled with ice water. In the first case, the radiometer will indicate warming, and in the second, it will indicate cooling (jokingly known as ‘cool radiation’).

2. The radiant intensity increases strongly with increasing temperature. This can be demonstrated using an electric cooking pot with a thermometer, which is placed as a “radiating emitter” at a distance of around 0.5 m from a radiometer that serves as “receiver”.