Light released within a sphere with a perfectly opaque inner surface

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antwan89
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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 isn't already the most opaque surface available, although I understand there heat losses occur when light is reflected off a mirrored surface.
 

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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.
 
than you mfb, but isn't 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?
 
No. "No light could escape" if the light is absorbed by the surface. "Opaque" does NOT mean "perfectly reflective".
 
antwan89 said:
A sheet of black paper is translucent is it not?
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.

If a truly Opaque material was used, surely no light could escape the sphere and it would remain permanently bright?
The light can get absorbed.