Ask A Scientist© Physics Archive

name        Russell
age         20s

Question -  We know that blackbody radiation produces an
intensity-frequency curve that is dependent on temperature. The idea of
course is that you are looking into the cavity through a hole. Why isn't
the outside of the blackbody also the same color as the inside (for a
given temperature)? And what it we had a blackbody made of sodium.
Wouldn't there be a peak in frequencies emitted that corresponds to the
traditional sodium spectrum? Is then the blackbody curve "material-specific"?
-Russell
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The blackbody curve itself is theoretical.  Some materials come close to it.
In practice, the radiation emitted from a cavity through a small hole
("cavity radiation") is very close to the theoretical blackbody curve for
the same temperature.  In the cavity, the radiation is essentially in
equilibrium with the material - most of the radiation stays inside the
cavity, being continually emitted and re-absorbed by the walls.  Radiation
emitted from the outer surface of a material will not necessarily be fully
thermalized - some frequencies corresponding to certain transitions of the
material, such as the sodium D line you mentioned, may be emitted
preferentially.  So, the blackbody curve is not material-specific, but the
actual emission from an object will be.  Cavity radiation will depend less
on the material, and the smaller the hole, the closer it will correspond to
the theoretical blackbody curve.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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A couple of comments:
     A "Blackbody" is a theoretical construct  which has a emissivity of 
exactly one
at all wavelengths.  This means that it absorbs all the energy that reaches 
it.  It
also means that its temperature is reflected in its emission spectrum (a 
Blackbody
curve).  The theoretical blackbody emission depends only on the 
temperature, not on
the material.  No material is a perfect blackbody.

     Laboratory blackbody emitters are approximations of a true blackbody that
consist of a heated chamber with a small hole to view the interior temperature.
This configuration is used because it is much easier to control the 
temperature of
the inside of an object than it is to control the temperature of its 
surface.  If
the interior of the chamber is at thermal equilibrium then absorption and 
emission
of photons are at equilibrium for every wavelength.  A small amount of energy
escapes through the viewing hole and must be replenished by providing 
electricity
to heat the chamber.  The escaping energy (light) has the same spectral
characteristics as the light inside the chamber -- which is a pretty good
approximation of the blackbody radiation.  The material of construction 
should not
vaporize easily.  Otherwise you will have cooler gas molecules around the 
exit from
the chamber and they will superimpose characteristic absorption lines onto the
blackbody spectrum.
Dr. Bradburn
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The blackbody is built as you say, a cavity with a hole.  The cavity is 
insulated,
so as to let it hold a high temperature (200 C to 1000 C, perhaps).  This 
can be a
small hole, maybe just the size of a quarter.  If the hole is too big, the 
cavity
gets cooled by air, or whatever, and doesn't do exactly what its supposed 
to do.

Spectral lines such as you mention are in the visible range, sodium is yellow
light.  The black body is mainly used as a source of infrared 
radiatiion.  It is
not typically used for visible light, as there are other sources 
there.  Plus the
black body would have to be very hot to emit in much in the visible.  If it 
were
that hot I don't think you would get much absorption right at the spectral 
lines
anyway, because the thing you describe would be made of solid sodium, not 
ionized.
The black body curve itself is not material specfic, but if there is some gas
between you and the black body then this would absorb.

but people make these things out of common metals.  Its just a big hot box.

Dr. Ross
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