This is off the subject a little, but is still about thermal radiation, although not "thermal" in physicist's sense of the word: The fires in Washington state. I heard indirectly through a friend in Little Rock that a couple named Patty and Freddie have read some of my blog posts, and they live in Washington in one of the areas experiencing the wildfires. I hope the fires are decreasing in intensity and will soon be under control and extinguished! Glad to hear the alpacas and other animals survived and came back home, and that your home is still there.
Back on the subject of a box of light, I haven't discussed whether there is air in the box or if it's a vacuum. I just have not even thought about that until recently. It isn't discussed in the books and articles I've read. But its pretty clear that our Planckian box of radiation does contain air, based on the difficulty and extra expense and lack of need to pump air out of the box. Oh, yeh, and the box has a hole in it! For viewing the spectrum of the light inside! So to be precise I must say that the Planck thermal spectrum box is not "empty" because it does contain air at atmospheric pressure.
Air itself can become incandescent when it's sufficiently heated. This happens with powerful bombs, both of the chemical type and of the nuclear type. The temperature of air surrounding the detonation briefly reaches thousands of degrees for chemical-based explosives and millions of degrees for nuclear detonations. For comparison, the temperature of the hydrogen and helium in the sun's photosphere is about 10,000 degrees Fahrenheit. These are all sources of thermal radiation, having approximately the Planck spectrum.
Getting back to the subject of fire, the flames we see from a fire are not due to incandescence. They're due to combustion of the material involved, and the type of material is what determines the the color of the flames.
