Light released within a sphere with a perfectly opaque inner surface

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Discussion Overview

The discussion revolves around the behavior of light within a sphere or cube that has a perfectly opaque inner surface. Participants explore whether the inner area would remain bright indefinitely, particularly in the context of a vacuum, and consider the implications of different surface materials on light retention.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions if a beam of light released within a perfectly opaque sphere would remain bright indefinitely, assuming a vacuum exists.
  • Another participant suggests that while opaque surfaces prevent transparency, they do not necessarily reflect light, and proposes that a perfect mirror would be required to keep light from escaping.
  • Some participants discuss the properties of superconductors, noting that they can act as nearly perfect mirrors for certain wavelengths, but still allow for some light loss over time.
  • There is a contention regarding the definition of "opaque," with one participant asserting that a truly opaque material would not allow any light to escape, while others clarify that opacity does not equate to perfect reflectivity.
  • Concerns are raised about light absorption by surfaces, indicating that even with opaque materials, light may not remain indefinitely bright due to absorption effects.

Areas of Agreement / Disagreement

Participants express disagreement on the definitions and implications of opacity and reflectivity, with no consensus reached on whether a truly opaque material could keep light from escaping indefinitely.

Contextual Notes

Participants highlight the limitations of definitions surrounding opacity and reflectivity, as well as the potential for absorption in various materials, which complicates the discussion of light retention within the sphere.

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.
 

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