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).
