"Bob (this one)" <[email hidden]> wrote in message
news:[email hidden]...
Quoted message said:That wouldn't happen in the dark. Light is radiant heat. It reflects
light. It doesn't reflect heat as heat, it just doesn't absorb radiant
energy well, nor does it transmit it. You say it reflects heat, so I'm
sure you'd closely wrap your hand in a single layer of foil and hold a
candle under it such that the flame was 1/2 inch below it. It'll reflect
the radiant heat (we call that light) but the conducted heat and
convected heat will fricassee your hand. That's why aluminum cookware
works well. It conducts heat very well; it doesn't reflect it back
towards the flame or the coil.
Sorry, Bob, I'm going to have to quibble a bit with the above. Light
is not "radiant heat," although we often speak of "heat waves" and so
forth as a convenient verbal shortcut. "Heat," technically, is the
mechanical motion of the atoms/molecules within a given substance;
such motion can be induced by several means, including the
absorption of energy from a "radiant" source, such as EM waves
(which is what light REALLY is). That's not quite the same thing as
"light IS heat." It would be more correct to say that "light (and other
forms of EM radiation) is a form of energy which can, among other
things, increase the temperature of a given object through being
absorbed by that object." In simpler terms, you don't really
"reflect heat" ever, you reflect the radiation which otherwise would
have been absorbed by the object in question, resulting in "heat."
That seems picky, but it is an important distinction here.
Anything that reflects radiation over a given spectral range will be
warmed by that radiation to a lesser extent than something which
absorbs more of that same radiation - a good example of this being
the difference between the temperatures of a white object and a
black object when placed in the sunlight. The white object obviously
is a better reflector of light over the visible spectrum than the black,
and so doesn't get as warm. The black object, on the other hand,
is absorbing more of the visible light, which means that it is converting
that energy to heat (the energy has to go into SOMETHING - it can't
just vanish).
Aluminum - or for that matter, most metals (especially when polished)
are actually excellent REFLECTORS of radiant energy (EM, and
specifically light), in addition to being excellent conductors of heat. In
the
example of wrapping your hand in aluminum foil, yes, you will "fricasee
your hand," but not as rapidly as you would if the foil in question had been
blackened (ideally, across the visible AND IR spectrums). A polished
aluminum
surface most definitely does "reflect heat." Holding your aluminum-wrapped
hand over a flame, though, brings other factors into play - it's not just
the radiation coming off the flame, but also the hot gases which are
rising from it and directly striking the surface. Perform this same
experiment in a vacuum - OK, we'll have to modify it somewhat,
with a purely-radiant source and something to also protect your hand
from being exposed to vacuum! - and the results are different. This is
why, for instance, reflective foils are so often seen covering the outer
surface of spacecraft and satellites - the ONLY way those receive
heat "from the outside" in space is via radiation, so the reflective
covering
is there to "reflect heat" (actually, to reflect the radiation - light, IR -
before it
gets absorbed and becomes heat).
So aluminum cookware, to get back to a specific topic relevant to
THIS group, DOES "reflect heat." It's just that the absorption of
radiation by cookware isn't the only, or necessarily even the primary,
means of heating said cookware. These things are also warmed by
the gases within the oven or whatever, and with that source, the
ability of the material to conduct heat is much more important. Still,
there will be a slight but measurable difference between a plain "silver"
aluminum (or other metal) container and one which is darker in color,
which is why you see such things as different cooking times/temps
on cake mixes when "dark" pans are used.
Quoted message said:You might want to consider why there are charts for conduction of heat,
but none for reflection of heat.
The reason for this is that the "reflection of heat" isn't an inherent
difference between these metals - i.e., aluminimum doesn't have
one specific "coeffiecient of reflectivity" which differs from that of,
say, silver. How well these materials reflect radiant energy is, for
one thing, too much a function of the surface finish for there to be
such tables. Also, there is, as previously noted, no such thing, t
echnically, as the "reflection of heat." What you WILL find (rarely,
because this information isn't commonly used in the selection of metals
for a given application) are curves of the spectral reflectivity of the
metal
over a given range of wavelengths (if limited to the visible range, then
such a curve would roughly be telling you "what color is this metal?"
although there's more to color than that, as well). A metal surface
which is a fairly good reflector of energy across the visible EM
spectrum (say, polished aluminum) WILL be heating by a purely-
radiant source much less than will one which is relatively poor (cast
iron, just to use the first example that comes to mind).
Looking back over your original comments, this may be pretty much
what you were intending in your discussion of "reflecting heat" -
but if so, it looked like you and the other poster were talking about
different things, or at least using the same words to point to very
different
phenomena. I hope this has cleared things up a bit. And sorry for
being so long-winded - it's just so rare that I get to post something here
which has to do at least somewhat with my particular field (I'm a display
technologist, so I get fairly heavily involved with such things as light and
color and such, not to mention thermal concerns in all the hardware!)
Bob M.
