Light released within a sphere with a perfectly opaque inner surface

[antwan89]

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.

 

[mfb]

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.

 

[antwan89]

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?

 

[HallsofIvy]

No. "No light could escape" if the light is absorbed by the surface. "Opaque" does NOT mean "perfectly reflective".

 

[mfb]

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.