Alexander Hollins wrote: > Molten glass at red stage is generally crystal clear. you can find > videos of glass blowing demenstrations on youtube and see for > yourself.
Thanks -- I'll have to look them up. Issue is that if it's radiating, say, 10% as much as molten platinum would (with roughly the same melting point), then it would produce a visible glow *and* appear transparent, particularly in thin section. This is squishier than I expected, so to speak (and candle flames turn out not to be *fully* transparent BTW but that's another story). Sigh .... I may have to get hold of a torch and do my own experiments here -- or find a good thermo textbook (ugh, I hated thermo in college, which is one reason I still don't understand it)... One interesting sidelight: Bill Beatty has posted at least one video in the past showing that hot glass turns opaque to microwaves. That was glass heated well below the glow point (until the microwaves hit it, of course) but that would make sense as microwaves are much lower frequency than light (obviously). > > On Wed, Jul 22, 2009 at 6:38 AM, Stephen A. Lawrence<[email protected]> wrote: >> I ran across an explanation of a "blackbody" which I actually understood >> a week or so back (totally unexpected, it was in the introductory >> chapter to a QM book), and since then I've been fiddling around with >> gedanken experiments involving black boxes with little holes in them and >> the second law of thermodynamics. >> >> And it appears to me that, according to the second law of >> thermodynamics, if glass is heated red-hot or orange-hot, and it's >> actually seen to be glowing orange, it should also turn *opaque* to >> visible light while it's at that temperature. (If its glow is weaker >> than, say, steel at the same temp then it should be semitransparent >> rather than totally opaque but none the less it shouldn't be >> water-clear, as it is at room temperature.) >> >> I've seen lead-crystal (very clear) glass being worked at high >> temperatures, at Corning many years ago, and as far as I can recall it >> did indeed glow bright orange. >> >> Does anyone here happen to know if glass also turns opaque (or >> semi-opaque) when it's heated to high temperature? (If it is I'll be >> amused; if it's not I'll have to go figure out where my reasoning went >> off the tracks.) >> >> I know for a fact candle flames are transparent, but I don't have the >> facilities to heat a pane of glass until it produces a cheery glow while >> shining a bright beam of light through it (don't even own a propane >> torch at this point, and in any case hitting a windowpane with a propane >> torch would probably shatter it). >> >> >
