On Jul 27, 2009, at 10:07 AM, Stephen A. Lawrence wrote:
Suppose you took a lump of glass and placed it in an (evacuated) oven.
Suppose further that the walls of the oven are dead black, absorbing
(nearly) all radiation which falls on them, and assume that they
radiate
about as you'd expect a blackbody to radiate.
Suppose further that the oven and the lump of glass are at the same
orange-hot temperature (and let's ignore the fact that the glass has
melted all over the bottom of the oven because that adds unnecessary
complexity to the experiment -- maybe we put the whole thing in
free-fall, or whatever).
Now the walls of the oven are giving off a cheery orange glow. Assume
the glass is glowing orange, too, and assume further that it's glowing
just as brightly as the walls of the oven. (This is an assumption; we
know glass glows *some* but we haven't confirmed that glass glows as
brightly as something which starts out black.)
I think there might be a misconception here about the difference in
behavior between heated surfaces and heated black body cavities.
Cavities, at least peep holes into cavities, act as almost perfect
black bodies. See:
http://en.wikipedia.org/wiki/Black_body
"In the laboratory, black-body radiation is approximated by the
radiation from a small hole entrance to a large cavity, a hohlraum.
(This technique leads to the alternative term cavity radiation.) Any
light entering the hole would have to reflect off the walls of the
cavity multiple times before it escaped, in which process it is
nearly certain to be absorbed. This occurs regardless of the
wavelength of the radiation entering (as long as it is small compared
to the hole). The hole, then, is a close approximation of a
theoretical black body and, if the cavity is heated, the spectrum of
the hole's radiation (i.e., the amount of light emitted from the hole
at each wavelength) will be continuous, and will not depend on the
material in the cavity (compare with emission spectrum)."
I know from personal observation that it goes beyond this. As a
cavity and its contents heat up, everything in the cavity eventually
disappears from view through the peep-hole. I have personally sat
and watched through a gas forge observation port, which I kept open,
the cover lifted, as that gas forge, which was about 1' by 2' by 2',
heated up. Initially, I could clearly see the far walls of the forge
and things in it through the port. When the temperature rose to an
orange glow, suddenly nothing was visible inside the forge. There
was a pure orange glow coming from the observation port that had
nothing to do with the contents of the forge. One moment I could see
the other side of the forge, which had some hot spots and dark spots
on it, and the next it was replaced by flat orange glow. I could see
nothing at all inside the port. It was as if the hole surface itself
(which is not a physical thing) was radiating.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
