Harvard Natural Sciences Lecture Demonstrations
Blackbody radiators in thermal equilibrium should emit the same spectrum of radiation; inside an oven at high temperature, objects should appear with the same color, whatever their material. A small ball of iron and a chunk of brick are placed inside a muffle furnace that has been preheated to about 1000 C. The interior walls and heating elements of the hot furnace have a bright, yellowish glow to them. The samples of iron and brick, on the other hand, are at room temperature (about 25 C) when they are first inserted, and so their brightness and color make them stand out against the hot background. The samples are heated for about 30 minutes, which is enough time for both of them to reach the same temperature as the interior of the furnace. When the door is opened, we see that the samples glow with nearly equal intensity and color as their surroundings! When the samples are removed from the furnace, we see that they both start out at nearly the same brightness and color. As they cool they become less bright, and the colors they emit gradually move toward the red part of the visible spectrum.
