blackbody radiation

A blackbody is an idealized object that soaks up every bit of light and heat that falls on it, reflecting nothing, and that, when warm, glows with a light of its own. Blackbody radiation is that glow. Everyday image: a lump of iron in a forge turns from dull red to orange to yellow-white as it gets hotter. The colour is telling you the temperature, and that is the question this topic answers: what light does a hot object give off, and why does the colour shift as it heats up?

Precisely, the shape of a blackbody's glowing spectrum depends only on its temperature, not on what it is made of. As the temperature T rises, the peak of the glow shifts to shorter wavelengths (Wien's law: peak wavelength is proportional to 1/T) and the total power radiated grows very fast (the Stefan-Boltzmann law: power per unit area = sigma T^4). Classical physics predicted the glow should carry infinite energy at short wavelengths, the so-called ultraviolet catastrophe, which is nonsense. In 1900 Max Planck cured it by assuming energy is emitted only in tiny lumps of size E = h f, where f is frequency and h is Planck's constant. That assumption was the first crack of the quantum age.

Why it matters: the Sun, a glowing light-bulb filament, and even the leftover glow of the Big Bang (the cosmic microwave background) all radiate almost exactly like blackbodies, so this curve turns up throughout astronomy and thermal physics. Honest caveat: a perfect blackbody is an idealization; real surfaces are somewhat 'grey' and emit a little less. Even so, the model is astonishingly accurate, and solving it is what set quantum theory in motion.