Peter wrote: > Olin Lathrop embedinc.com> writes: >>> It's blackbody radiation, and everything emits it and absorbs it. >> No, only truly black bodies do. Everything else emits and absorbes >> less. > > Less is true, but not by much. Melanine, ... I was talking about physics here, not specific pigments (and it's melanin, melanine is a kind of plastic). Also, the less than black body can be substantial. Common white paper absorbs only about 15% of the visible light that a truly black piece of paper would, for example. And many things are more reflective than common white paper. > Actually gases and transparent objects in general *do* emit blackbody > radiation like everything else. Transparency and emissivity are two > almost unrelated properties, Completely wrong. Go back and check your freshman physics. Put in terms you may be more familiar with, any antenna works equally well or poorly whether receiving or transmitting. There is no such thing as a one way antenna. This doesn't change when the wavelength gets a bit shorter. Think about it. If this weren't true you could violate thermodynamics and build free energy machines. Put any object that emits differently than it obsorbs into a vacuum inside a glass housing on your desk. If it absorbed more than it transmits, then its equillibrium temperature would be higher than the surroundings. If it transmitted more than it absorbed, its equillibrium temperature would be lower. Either way you'd be getting free energy when there isn't any to be had. Air, like everything else, emits IR just as well or poorly as it absorbs it. Since air is pretty much transparent to IR, it can't be radiating it either. It will radiate a tiny amount, due to impurities like dust and to the extent it's not fully 100% transparent. > as you have worked with computer > graphics you should know that (in extremes one can generate computer > models of 3d ray traceable lights and halos that are transparent yet > emit quite fine). This means nothing of course. I can do all sorts of things in computer graphics that violate physical reality. Just because you can draw a picture of something doesn't make it real. Do you believe in Homer Simpson and Sponge Bob talking underwater too, let alone a sponge with arms sticking out speaking english? ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist