Can refraction of light change its entropy?

  • 4 Replies
  • 3760 Views

0 Members and 1 Guest are viewing this topic.

*

Offline Atomic-S

  • Hero Member
  • *****
  • 945
    • View Profile
Can refraction of light change its entropy?
« on: 30/07/2009 00:21:52 »
If a sunbeam is passed through a sheet of crinkly glass, is the entropy of the light greater than, equal to, or less than it was previously?
 

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Can refraction of light change its entropy?
« Reply #1 on: 30/07/2009 04:58:16 »
How do you propose to measure the entropy of light?  From a classical optics point of view, the light becomes less coherent, which means it is more random, and increased randomness means more entropy.

*

Offline Atomic-S

  • Hero Member
  • *****
  • 945
    • View Profile
Can refraction of light change its entropy?
« Reply #2 on: 04/08/2009 07:03:13 »
I would begin measurement by asking how much work can be extracted from the light. Diffuse light can generate electricity in photovoltaic panels, but direct sunlight can also be focused to a high temperature, which presumably can generate more power. So I guess we have to get quantitative about that, about how much power is potentially available from each method.
 

*

Offline Atomic-S

  • Hero Member
  • *****
  • 945
    • View Profile
Can refraction of light change its entropy?
« Reply #3 on: 10/08/2009 06:46:19 »
Another way to look at this is, what is the highest temperature that can be reached by focusing the light upon a target? It is evident that light from, say, a clear incandescent bulb can be focused more sharply than that from a fluorescent tube of the same lumen output, so that a higher temperature upon a target will be reached with the former. This implies, thermodynamically speaking, that this light can be converted by such a process more efficiently into work. For a correct analysis, we must idealize the situation to the maximum theoretically possible concentration, impinging upon a perfectly absorbtive targer (i.e., black body). Then dS = dQ/T .
 

*

Offline Atomic-S

  • Hero Member
  • *****
  • 945
    • View Profile
Can refraction of light change its entropy?
« Reply #4 on: 10/08/2009 06:48:11 »
So how would we do the optics for this? That gets a bit sticky, dealing with a non-point source. A first approximation would be to use a deep parabolic reflector having a round fuzzy black object centered on the focus, and the smallness of the object will be limited by the extent to which the light becomes concentrated near the focal point.