Radiation in Perfectly Insulated Container
A black and a white body will be heated up to 600 °C. and then put out of the furnace into an isolation. This isolation stops the exchange of thermal energy with the surroundings. All energy, i.e. heat and radiation, therefore remains in the two bodies resp. in the space in between. Through this space the two bodies can exchange energy by radiation.
Now a blackened surface emits the maximum of radiation and at the same time absorbs all incident radiation. A white surface absorbs less radiation, but also emits equal less of it.
Both bodies stay in an equilibrium of radiation and have a certain temperature, namely 600 °C.
If the exchange of thermal radiation between both isolated bodies would happen as shown in the figure at the right, more energy would be transferred form the black to the white one. The white on will heat up and the black one will cool down. Thus thermal energy would flow form a cold to a hot place without any input from the outside. But this is impossible.
What exactly happens between these two opposite surfaces of the bodies?
- a) The black body „controls “ its radiative activity down to that of the white body.
- b) Each body absorbs its own radiation, emitted just before.
- c) The amount of radiation from the black body which is not absorbed by the white body will be reflected back to that. So they are again in an equilibrium of radiation.
The answer is: c)
As both bodies have the same temperature and are located in a closed system, an equilibrium of radiation will finally be reached. What is missing in the above figure is the part of the radiation form the black body which is reflected at the white one.
…problematic with description of radiation of a “black body” end of 19th century
…Planck’s solution…energy quantum…