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Effect of a distant cold object in focus of concave mirro.

  1. Feb 6, 2016 #1
    Pictets 1790 experiment shows the apparent reflection of cold from ice in the focus of a concave mirror onto an air thermometer held in the focus of a second concave mirror 12 feet away. As soon as the screen shielding the 'cold' concave mirror is moved aside the distant thermometer in the focus of the other concave mirror begins to descend under room temperature and remains there until the cold source is hidden again.

    http://www2.ups.edu/faculty/jcevans/Pictet's experiment.pdf

    I am not asking here why the thermometer reading falls.

    I am asking here what actual effect are these concave mirrors having upon the experimental results? Does the use of these concave mirrors force us to conclude anything particular about the observed results that in any way cause us to be concerned about what we are observing?

    Once again i am not asking why the thermometer becomes colder. I am just asking how we should interpret the useage of the two concave mirrors to get this result.
     
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  3. Feb 6, 2016 #2

    Dale

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    They focus the radiation from the ice onto the thermometer and vice versa.

    I wouldn't use the words "force" or "concern", but the use of the mirrors in the experiment provides a strong hint that radiative heat transfer is mediated by a form of light.
     
  4. Feb 6, 2016 #3
    Thanks. I am fine with the results and how they can be used if the results can be accepted at face value and the experiment is in no way cheating. I do not want to say more at this point in time in case what I say changes the answers I am given.
     
  5. Feb 6, 2016 #4

    russ_watters

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    Since you're looking for more answers, I'll just say that there isn't anything unusual/surprising/concerning here; Dale's answer was fine.
     
  6. Feb 6, 2016 #5
    Thanks. Are there any issues about magnification and if there are none how would you frame your answer so that the most challenged of people can understand there is no magnification?
     
  7. Feb 6, 2016 #6

    SammyS

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    What does the paper you referred to say about the effect of the mirrors?
     
    Last edited by a moderator: May 7, 2017
  8. Feb 6, 2016 #7

    russ_watters

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    I don't know what you mean by "magnification". The mirrors enable capturing a larger cross section of the radiation for emission/absorption.
     
  9. Feb 6, 2016 #8
    Thanks. Can you expand on what you mean by "capturing a larger cross section", so that it is clear what you mean in comparison with could be potentially achieved by other means if convection could be minimised to the same degree as the mirrors enable.
     
  10. Feb 6, 2016 #9

    russ_watters

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    An object radiates toward another along a direct line of sight. The subtended angle is based on the cross section and distance. If you have a larger object, the subtended angle is larger. That's what the mirror provides.

    Another way to think about it is that an object radiates in an expanding sphere. The mirror traces-out a large section of that sphere's surface and captures the radiation that hits it.

    I'm not sure what you are referring to when mentioning convection or "other means" here. Convection is not a factor here. And what "other means" are you referring to?
     
  11. Feb 6, 2016 #10
    What I am asking is whether or not the mirrors enable something to be done in the room that could not be done in the room by some other means such as exposing ice cold walls and so forth where such methods would somehow have to deal with the convection to get reliable results. We could for example expose an ice cold floor.

    Please bare in mind this experiment was difficult to set up and repeat by others and at the time it created a revolution in our understanding of heat.

    Count Rumford who later did a series of experiements with the radiation of cold, without using mirrors, wanted to avoid "concentration" by means of concave mirrors.
     
    Last edited: Feb 6, 2016
  12. Feb 6, 2016 #11

    russ_watters

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    I still don't understand. You said "something" and "somehow". Can you not be more specific? Are you suggesting with that last sentence that the results were falsified by purposely adding the confounding factor of a cold floor?
     
  13. Feb 6, 2016 #12
    Not at all. i was suggesting a possible alternative way of doing the experiment without mirrors where suddenly exposing the still air of a room to large areas of cold at a distance is going to create all manner of atmospheric disturbances in the room and is going to need a method to avoid disturbing the air.

    i just need to be totally confidant that use of the mirror achieves the same sort of result without resorting to trickery or fakery such as some kind of magnification or whatever it might be.

    Count Rumford who later did a series of experiements with the radiation of cold, without using mirrors, wanted to avoid "concentration" by means of concave mirrors.
     
    Last edited: Feb 6, 2016
  14. Feb 7, 2016 #13

    sophiecentaur

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    It is hardly surprising when the experiments of around 250 years ago and their descriptions are difficult to reconcile with modern accepted ideas. Count Rumford was on the cusp between Caloric and Modern theories of heat so what he wrote has been 'cleaned up' a lot for presentation to modern students. We are looking back through a very complicated channel - great for Science History but not necessarily for Science Understanding. (Just try reading Newton's original wording of his laws of motion, for another example.)
    This 'explanation' of the situation doesn't involve any trickery of fakery.
    A disc of ice will provide the same source and sink of heat as a mirror in the situation described if the two diameters are the same. The equilibrium temperature of the thermometer will be achieved when the net heat absorbed and radiated by the thermometer is equal to the net heat absorbed and radiated by room and ice. (Including convection and conduction is really not worth while in an initial approach). The heat flow into and out of the mirror (and its focus) will be due to the temperature difference between the thermometer and the ice times the solid angle subtended by the mirror. There is an Image of the ice block appearing at all angles within the mirror. The rest of the solid angle sphere is at room temperature and there will be a similar heat flow balance. The final thermometer reading will be an average of the two temperatures (weighted by the relative areas.)
    You could, if you wanted, describe the effect as a 'magnification'. Certainly, if you look at an object just less than the focal length of a concave mirror, with your eye just greater than that distance, you will see a massive image, spread all over the mirror. In each direction you look, you see a different part of the image and your thermometer will see a different part of the ice block in all parts of the mirror. I don't really like the word `"magnification", though because it could imply that something more is going on.
     
    Last edited: Feb 7, 2016
  15. Feb 8, 2016 #14
    Thanks for all of the answers.
     
  16. Feb 8, 2016 #15

    sophiecentaur

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    There was a post about a "cold mirror", which is an interesting idea because it introduces a 'real' factor into the problem. An ideal mirror reflects everything that falls on it and has Zero Absorptivity. Hence, it also has zero emissivity and can be treated as if it isn't there, thermally. Whatever temperature it happens to have, it will not emit or absorb any radiation from its surface. A real mirror will have some emissivity so its temperature will be relevant. You can replace it (treat is as equivalent to) with an ideal mirror and an almost transparent film, in front of it, with a finite emissivity. So it will absorb / emit a finite amount of thermal radiation. By splitting the model into two, you can come up with a prediction about what would happen. It will be like a mirror with the same area as before and a fine grid, over it, with whatever temperature you care to give it. IF its temperature is 0°C, the situation will be exactly the same as with a perfect mirror and the remote ice cube. With the mirror at any other temperature, you just add in a third 'body ' into the problem.
    Apropos of the same idea, the reflector of a radio telescope will be at around 300K but it is still worth while having the receiving amplifier (at its focus) cooled to around 50K because the thermal noise contribution from the reflector is almost negligible, despite being much 'hotter'.
     
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