Does Optic Focusing Contradict the Second Law of Thermodynamics?

[the4thamigo_uk]

I guess I am just a little dissatisfied with the explanation is all... I can't see what is wrong with it?

 

[the4thamigo_uk]

So you can't have all the heat transfer from i go to j and have all the the heat transfer from j go to i. For your BB example, the view factor from the BB to an enclosure would be one as you said. However, the enclosure would be at least partially radiating to itself thus the view factor would have to be less than one.

Ok yes I see your argument now. It comes down to the fact that there is a spread of rays in all directions from each point on the inner surface of my enclosing BB, so my simple calculation is clearly flawed as it assumes radial rays. So some will miss the surface of the inner body and be reabsorbed by the enclosing BB. I wish someone would have put it in those terms... but yes that was an excellent post.

An interesting one and I stand corrected!... Is there a 'general proof' of this for all geometrical arrangements?

I very much enjoyed it though and I learned quite a bit too... Youve got to keep questioning and pushing in order to get a good understanding of something. Thanks for your patience everyone... I will write this up and stick it on the internet somewhere.

 

[Roger44]

So, I was apparently wrong when I challenged Mgb Phys who said :

"For example, you can only heat a blackbody target using a blackbody source to the same temperature as the source. So however much you concentrate the light with lenses/mirrors you can't use the sun to heat something to a higher temperature than the surface of the sun. Because at that point the target would emit back at the sun."

I stand corrected but no regrets because I very much enjoyed the debate too. Thanks everbody for these two days of pure pleasure.

 

[the4thamigo_uk]

I stand corrected but no regrets because I very much enjoyed the debate too. Thanks everbody for these two days of pure pleasure.

If you want some more fun have a look at my thread on stefans law...? See where it might lead?

 

[the4thamigo_uk]

I am writing a few notes on this but a thought came to me. Please humour me once more...

It seems that the truth of the 4 laws, rests on their ability to prevent nonsensical situations arising in the world. It also lies in their consistency, meaning that any deductions we make from them must not be self-contradicting. Finally it lies in their completeness at describing the world. We as humans, observe the world and extract from it these basic postulates, axioms or laws. The 4 laws don't explicitly mention geometry, but since they have been extracted from empirical evidence about the real world, so they necessarily have geometry embedded deep down in them somewhere/somehow.

So, if we come across some imaginary geometrical experiment (like we have seen), that appears to contradict the 4 laws, can we 100% always immediately say on the basis of the laws that this imaginary experiment cannot exist in reality. In other words does a fundamental proof lie in reference to the 4 laws, or in reference to a more complete understanding of the geometry?

I now understand that the Stefan-Boltzman law can be derived purely from the 4 laws (http://en.wikipedia.org/wiki/Stefan–Boltzmann_law), so there is no new essential physics contained within it. Its just a convenient formula to work with and make calculations easier. So it would appear to me now that a contradiction of the 4 laws is sufficient to disprove a given experiment.

So it looks like there is no need to 'prove for all geometrical arrangements that you cannot have all the radiation transmitted from one BB to another BB'. If you ignore geometry completely you get the second law contradiction and that is sufficient argument, *because* contradiction of the second law leads to absurdities that are not observed in nature.

I know that's what you were saying Sylas, Yeti but it takes some time to come to terms with it. I haven't studied this stuff since my college days.

Sorry if its a bit philosophical... Thanks again everyone

 

[Nabo00o]

Sorry I know this is a bit old, but I just got to say; they got to you.
Optic focusing 'will' contradict the second law, but you nor nobody else wants to face that burden...

 

[Mapes]

Sorry I know this is a bit old, but I just got to say; they got to you.
Optic focusing 'will' contradict the second law, but you nor nobody else wants to face that burden...

Probably because this argument has been rehashed many times, including in the physics literature. See, for example, S. Panse ("Non-spontaneous radiative heat transfer," J. Phys. D: Appl. Phys. 25 (1992) 28-31), which claimed to present an optical focusing system that contradicted the Second Law, and K.M. Browne ("Focused radiation, the second law of thermodynamics and temperature measurements," J. Phys. D: Appl. Phys. 26 (1993) 16-19), which refuted Panse's argument and showed that in fact the Second Law was not violated in these systems because the finite size of the radiating body prevented focusing its rays to a point.

If you think you have a better approach that avoids Browne's refutations, publish it.