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David Hall
2001-Dec-02, 09:12 PM
I feel like this is a stupid question, but all of a sudden I'm not sure about what a 'black body' object is. I remember learning about it in my university astronomy class, and I thought I understood it, but from some of the posts here, my understanding doesn't seem to fit with what I'm reading.

I always thought it was an object that basically "absorbed" or lets pass all radiation that hits it, so that it's spectrum doesn't show any reflected light. But some things I read don't seem to fit this definition. Are there other ways to use this term, or am I getting something wrong here? Or maybe basically have it right and I'm just confusing myself unnecessarily? Please educate me on this subject. /phpBB/images/smiles/icon_smile.gif

Wiley
2001-Dec-02, 09:44 PM
The ideal black body will absorb all incident radiation; this is just the opposite of "let pass all radiation." The ideal black body also emits radiation, the maximum radiation for a given temperature. Hence the term black body radiation.

In practice any object that does not reflect or transmit light is a good approximation to a black body. The canonical astronomical example would be a star.


Here are a couple of links that go into more detail:

http://www.phys.virginia.edu/classes/252/black_body_radiation.html

http://www.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html

David Hall
2001-Dec-03, 12:25 PM
Ok, thanks. Obviously I got a little confused by the 'pass through' part, but basically I was right.

I think what really confused me was when someone would say something like the cosmic background radiation is a black body spectrum, or something like that. I'm still a bit confused as to how something emitting at a single wavelength can be a be a black body, but I think I understand a bit better from the links you provided.

Bob
2001-Dec-03, 02:48 PM
On 2001-12-03 07:25, David Hall wrote:
Ok, thanks. Obviously I got a little confused by the 'pass through' part, but basically I was right.

I think what really confused me was when someone would say something like the cosmic background radiation is a black body spectrum, or something like that. I'm still a bit confused as to how something emitting at a single wavelength can be a be a black body, but I think I understand a bit better from the links you provided.



A blackbody emits radiation at all frequencies, not just one, in a characteristic curve of energy vs. wavelength for different temperatures. Classical physics could not explain the shape of the curve, especially at high frequencies ("the ultraviolet catastrophe"). Planck's major contribution was to derive the shape of the curve. But to do so he had to assume that the emitted energy was quantized.

Donnie B.
2001-Dec-03, 06:50 PM
On 2001-12-03 07:25, David Hall wrote:
I think what really confused me was when someone would say something like the cosmic background radiation is a black body spectrum, or something like that. I'm still a bit confused as to how something emitting at a single wavelength can be a be a black body, but I think I understand a bit better from the links you provided.



The simple answer is, the cosmic background is not a single wavelength, but a spectrum of energy over a range of frequencies. That spectrum approximates the ideal black-body curve to a high accuracy.

That's why the cosmic background is usually described as "4 degree Kelvin" radiation, rather than a particular RF wavelength. {Note: the temperature given above is a one-significant-digit approximation /phpBB/images/smiles/icon_smile.gif )

ljbrs
2001-Dec-04, 01:34 AM
Those were great links. Thanks.

ljbrs /phpBB/images/smiles/icon_biggrin.gif

Aodoi
2001-Dec-04, 09:13 PM
A thousand pardons, bad joke approaching: Hali Berri is the best example I can think of...