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Light released within a sphere with a perfectly opaque inner surface

  1. Jul 29, 2014 #1
    I have a question that has been puzzling me. If a beam of light was released within a sphere or cube with a completely opaque inner surface would the inner area remain bright for a prolonged period of time or perhaps indefinitely assuming a vacuum exists within the sphere?

    Perhaps a perfectly opaque surface does not exist? Perhaps a mirror made from a superconductor would be a more effective inner surface assuming the mirror isnt already the most opaque surface available, although I understand there heat losses occur when light is reflected off a mirrored surface.
     
  2. jcsd
  3. Jul 29, 2014 #2

    mfb

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    Opaque (=not transparent) is not enough, a black sheet of paper is opaque as well. If the surface is a perfect mirror, then sure, the light would stay in.

    Superconductors are extremely close to perfect mirrors - with microwaves, you can "trap" light for ~100 milliseconds with them (source), and the radio-frequency cavities used for particle accelerators achieve similar Q-factors as well.
     
  4. Jul 30, 2014 #3
    than you mfb, but isnt opaque also defined as "Impenetrable by light; neither transparent nor translucent." A sheet of black paper is translucent is it not? To the human eye it will block out most of the light, but some will escape.

    If a truly Opaque material was used, surely no light could escape the sphere and it would remain permanently bright?
     
  5. Jul 30, 2014 #4

    HallsofIvy

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    No. "No light could escape" if the light is absorbed by the surface. "Opaque" does NOT mean "perfectly reflective".
     
  6. Jul 30, 2014 #5

    mfb

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    That's not the important point (add multiple layers if you like). A black surface does not reflect light, even if the material behind it does not allow any light to pass.

    The light can get absorbed.
     
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