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Many, fortunately not all, discussions of the blackbody problem begin with trying to get us to imagine a doohickey called a blackbody and are accompanied with illustrations of odd devices supposedly like a blackbody, but none of them are perfect. And it is hard to figure out what they are getting. So, the approach here is just to say "The Blackbody Problem" is the name of this problem, and do not worry about what a blackbody might be if any existed.
At least from the invention of fire, perhaps so far back as the first human survived seeing a lava flow, people have observed that very hot things glow. Moreover, among the very hot things, the hottest look white, while the not-so-hot glow a ruby red, and in between are the brighter shades of red and the yellows. (The sun glowed white, too, but it looked the same on hot days and cold days, so the sun probably was not the origin of this observation.)
Eventually, people — at least blacksmiths and glass blowers — learned to gauge high temperatures rather well simply by the color of the light that hot stuff emitted. When English came along, people would speak of a "white hot rage," when the color associated with anger was red. They meant white hot was angrier than angry, because white is hotter than red. Everybody knew that as much as they knew apples fall out of trees.
As classical (before the Twentieth century, physics had got at the falling apple, eventually it got to the very hot stuff.
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