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WMAP map of the CMB. Credit: WMAP team
Because blackbody radiation depends only on the temperature of the black body doing the emitting, blackbody radiation is an example of thermal radiation (EM radiation due to the heat of the object, not that which is reflected or transmitted).
If you plot the intensity of blackbody radiation against wavelength (or frequency), you get a curve with a very characteristic shape – it has a long tail (on the low frequency side), a well-defined peak, and a sharp drop (on the high frequency side); no surprise to learn that it is called the blackbody spectrum!
The blackbody spectrum was intensely frustrating, yet also intriguing, to 19th century physicists … because they could not explain it. In a breakthrough that heralded the quantum revolution of the early 20th century, in 1901 Planck derived an equation – called Planck’s law – for the blackbody spectrum, using an assumption about the quantization of energy; prior to this Wien had accurately described the short wavelength side (the steep drop), and what is now called the Rayleigh-Jeans law described the long wavelength side (the long tail). A great many books on the history of physics make the mistake of giving Planck credit for photons, the quanta of EM radiation; in fact, it was Einstein who first applied the idea of quantization to light (in the photoelectric effect, this is what he got his Nobel Prize for), and not until 1924 did Bose ‘close the loop’ by developing photon statistical mechanics (from which Planck’s law can be derived, ‘from first principles’).
No physical object is a black body; apart from some residual reflection (nothing – except a black hole – is totally black), the structure and composition of real objects leave their marks on the emitted radiation, even at very low temperatures: atoms and molecules emit and absorb at preferred wavelengths (lines and bands, respectively); the electrons in metals behave as a Fermi gas; in semiconductors the band gap(s) produce characteristic distortions; … and irregularities on the surface – pits, ridges, etc – can give rise to interference effects.
All the more exciting, then, when something with a spectrum approximating a blackbody far more closely than anything seen – or made – on Earth was discovered … by astronomers; the cosmic microwave background (CMB). And why does it have so nearly a perfect blackbody spectrum? Another Guide To Space article explains why.
Want more? Black Body Radiation, by Michael Fowler of the University of Virginia, is part of a 200-level university course , MyBlackBodyApplet (the name says it all), another one (with cross-reference to stellar spectral types), and Blackbody Radiation & Wien’s Law, from NASA’s Imagine the Universe!
Blackbody radiation is mentioned in only a few Universe Today stories (other than those on the CMB!); an example: Spitzer Discovers Early Galaxy Forming Region.
Energy Levels and Spectra, an Astronomy Cast episode, puts blackbody radiation into a broader context.
Sources: Hyperphysics, NASA
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