The formation of blackbody radiation

[jauram]

I am puzzled about the formation of blackbody emission (Planck's law). Specifically, we know that such things like incandescent lamps, an electric arc in a gas at high pressure etc. produce a nearly blackbody spectrum of corresponding temperature. Does this mean that in these cases a nearly equilibrium state between radiation and matter is set up? Or how do you explain why the spectra are close to that of a cavity?

 

[lalbatros]

Consider first a cavity with a small hole: the emission is low enough as to not perturb the BB radiation.

Consider now a very large volume of gas. Since the volume is very large, most of the radiations emitted inside the volume will be reabsorbed, only a tiny fraction has chance to escape.

Consider now a small volume of a solid.
This is exactly similar to the large volume of gas, as long as this volume of solid does not become transparent (like a gold foil).
That's why hot iron from a blast furnace also shows a nice BB spectrum.

The key point is "transparency".
See the full theory of radiation heat exchanges for more details.
These notions of emissivity, absorptivity have many applications, for example in combustion modeling.

Michel