May 302016
 

So. You have yourself a mirror, and you have yourself a ridiculous bright source of light. Maybe it’s a laser and an aiming mirror, maybe it’s the sun and a solar sail deployed at a dangerously close perihelion. Since no mirror is 100% reflective, some of the light is absorbed and the mirror temperature climbs. Assume the mirror is made out of something that can take very high temperatures (say, iridium)… so high that the mirror becomes incandescently hot without melting. So, the question is… if your shiny iridium mirror is glowing white hot… is it still a  reflector? I know that with gases, the most transparent vapor become a near-perfect absorber of incoming radiation if it’s incandescent. But how about a solid reflector?

 Posted by at 9:54 am
  • First-Light

    If the material glows in the visible, it is no longer a reflector – it is an emitter.

    • Scottlowther

      But what happens to the light impinging on it? Does a laser beam scatter rather than reflect? Or is it absorbed?

      • First-Light

        That’s a little more complex – if the laser is at a frequency that is within the emission, the light can be scattered (but problematic to measure) but probably not reflected, if the laser frequency is outside the range of the emission, then the light can be reflected.

        • Scottlowther

          OK, that’s kinda what I expected. So if you have an Insanely Hot Glowing Thing and a parabolic mirror used to direct the light, you’d better make sure the mirror doesn’t get to glowing in visible, or it’ll stop reflecting and itself become an Insanely Hot Glowing Thing (though obviously not quite *as* hot as the source).

          • First-Light

            Yes, that’s basically it – the surface properties of a material change when you get the thing to glow. If its glowing – and you probe the surface with a photon of the same frequency ans the glowing light – how do you separate the probe photon from the emission photon?

          • Scottlowther

            Well… if you have a flat plate glowing yellow-hot, you’ll have light (with the top of the bell curve distribution in the “yellow” area, IIRC) emitted more or less uniformly. If the plate *isn’t* glowing and you point a yellow laser at it, some distance downrange you’ll find a reflected spot of yellow light. But if the flat plate mirror is glowing *and* has a laser pointed at it, the downrange target might not glow very much from the emitted yellow light, but if it’s still reflecting you’ll clearly see the yellow laser-spot. But if the mirror *absorbs* the laser light, it’ll simply turn that energy around and re-emit it more or less uniformly. So if the laser is heating up the mirror, I’d expect you’d see the yellow spot downrange start off bright and then either fade away or get fuzzy and disperse or something. But if the mirror continues to reflect while glowing, the downrange spot will remain.

          • Paul451

            Try asking on physics-stackexchange? Larger pool of readers.

            My gut instinct is that once the surface is emitting at a range of frequencies that overlaps the laser, the material will absorb that frequency too. If particles are emitting scattered light, its hard to see that they’d be able to retain the quantum information from reflected light, so they’d absorb/re-emit like a white non-mirrored surface.

            But if the frequency of the laser is outside the range being emitted, the mirror would still reflect normally (plus/minus the effect of the heat on the mirror, obviously.) After all, a mirror at room temperature is still absorbing energy from the room and emitting infra-red.

            Random thought or plot point in a story?

          • Scottlowther

            Kinda neither…

            A minor point in a bit of non-fiction (sorta) that I’ve been poking away at.

          • publiusr

            I’d go to cosmoquest.org–phil plait’s old site. Similar topic some time ago.

          • publiusr
  • Peter Hanely

    Being a reflector means light is absorbed and emitted poorly at the frequency reflected. If our mirror remains reflective at a few thousand K it will emit light from incandescence poorly in proportion to how well it reflects. It would be a good idea to make the back side of the mirror a radiator if you expect enough incident light to heat it up.

    As for how reflectivity of any given material changes as it reaches such temperatures, I have no idea.