Black Body Radiation

January 15, 2009 8:53 AM

What can you tell me about black body radiation?

Each week I have been trying to learn about a physics concept with a friend.

After we research on our own, we plan to get together to share what we have learned.

What interesting, exciting, important, etc. information/revelations can you share with me about black body radiation?

posted by PaulingL to science & nature (8 comments total)

This is kind of a broad question. Have you read up on this? Do you have any specific questions?

That said, the classical theory of Black-body radiation will lead you to the Ultraviolet catastrophe which led to acceptance of Planck's Law which led to...well, quantum mechanics.
posted by vacapinta at 9:05 AM on January 15

Vacapinta hit the big picture things very well. The Ultraviolet catastrophe showed that somehow, the physics in it weren't continuous ... you didn't have an infinitely smooth range of temperatures, there were actually tiny little steps. Imagine the difference between integrating a curve in calculus and Simpson's rule. The fact that the universe wants to do these tiny little steps meant that things aren't always in the set of real numbers, just a finite subset. Weird!

Here are some fun little bits:

A theoretical black body absorbs and throws out radiation. A poor black body is hard to heat via radiation, and cools by radiation off slowly.

A star is an approximation of a black body.

The way the "hump" of the blackbody radiation curve works, combined with the way the human eye works, means that we see stars which are red, orangey, yellow-white, and blue-white, but never green. That always bugged me as a kid.
posted by adipocere at 9:19 AM on January 15

A star approximates a black body as adipocere says, but another astronomical source - the CMB is the closest thing to a black body that's naturally occurring.
posted by edd at 9:26 AM on January 15

Black body spectra lie behind the theory of Colour temperature, and hence into much of the colorimetry of graphic design, film, photography, video, etc.

As vacapinta said, it was one of the leading unexplained effects that led to quantum mechanics and modern physics.

Following on from the stuff about stars, treating the sun as a black body radiator allowed the (surface) temperature to be established as some 6300K - which ruled out a lot of 19th century theories about what the sun was made of (I seem to remember a mass of burning coal being one more eccentric theory) and pointing to the requirements for some then not-understood energy source (ie. nuclear fusion.)

The Cosmic Microwave Background radiation matches the spectrum for a black body to a crazily high precision, one of the principle pieces of evidence for the Big Bang theory.
posted by Luddite at 10:19 AM on January 15

Another tidbit: blackbody radiation carries the maximum possible amount of entropy that radiation can carry. (Entropy corresponds to a system's number of possible microstates, or configurations that are equivalent for the purposes of bulk measurements like temperature or pressure.) The converse is a perfectly coherent, monochromatic, polarized beam, which occupies a single microstate. As a result, when considering types of energy transfer (i.e., heat vs. work), blackbody radiation is pure heat transfer.
posted by Mapes at 10:27 AM on January 15

Adding to adipocere: one star with an intensity peak in the green is the sun (5800 K, 500 nm).
posted by fantabulous timewaster at 5:29 PM on January 15

it was pointed out, in a book i was reading, that the sun's blackbody spectrum peaks at almost exactly the wavelength of the deepest minimum in the absorption coefficient of water. this is a fairly important thing, in terms of the suitability of our water-drenched planet for the evolution of life forms that all ultimately rely on the sun for energy by photosynthesis.

taken in terms of the whole electromagnetic spectrum, or even the UV-IR range, these are two relatively narrow windows. the implications of and reasons for the coincidence of the two things might be something fun for you to think about.
posted by sergeant sandwich at 6:43 PM on January 15 [1 favorite]

oops, meant to link to this: water absorption spectrum.
posted by sergeant sandwich at 6:44 PM on January 15