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Thread: a Black Sphere

  1. #1 a Black Sphere 
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    Myself and many colleagues got into a long discussion yesterday, whereby many different suggestions were giving regarding the most probable outcome to the below scenario.
    I would like hear the opinions of those that work in the scientific field.


    Imagine having a sphere which is completely 100% transparent on the outside but completely 100% reflective on the inside ( if you chopped the sphere in half, then you would see straight through it from the convex side, whilst if you looked from the concave side you would see a reflection of yourself).

    You hold the sphere with the above properties in front of you , what will you see when you look into the sphere?

    I believe that all you could ever see is a constant black. This is because the light would enter into the sphere, but because the interior is 100% reflective the light could never then leave the sphere. Thus you would never actually be able to see what is inside the sphere because the light rays inside it would never hit the back of your eyes. Hence all you would see is black. That is until there is far to much energy inside the sphere for it to be contained...

    Would you be kind enough to share your opinion on such a scenario, and why you have come to the conclusion which you have.

    Thanks

    Mike.


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  3. #2  
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    I would like to appologise for my lack of scientific knowledge, and i suspect that this is possibly the reason why my post has not recieved any replies.

    In order to help myself construct better posts which will attract more responses, would someone please take the time to explain what is wrong with my original post?

    I would greatly appreciate any feedback.

    Regards

    Mike Malinowski


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  4. #3  
    Universal Mind John Galt's Avatar
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    Never apologise for lack of knowledge. Only apologise for claiming more knowledge than you posses. This you have not done.

    On the face of it the obvious answer, the one you have come up with, appears correct. It may be, but I am always suspicious of the obvious answer. Here is another possibility to consider:
    If we are admiting photons to the interior of the shell, then we are increasing its internal 'store' of energy. My understanding is that photons do not reflect off of a mirror in the same way a ball bounces of a wall. Rather the photon is absorbed, raising the energy level of an electron, which almost immediatley drops back to its prior - or sometimes lower(?) - energy level. This process, on average, imparts some energy to the mirror, so that its temperature rises. Consequently it will begin to radiate, becoming visible in the infra-red after some finite time.

    But I'm not a physicist, so that might be bollocks.
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  5. #4  
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    Quote Originally Posted by Ophiolite
    My understanding is that photons do not reflect off of a mirror in the same way a ball bounces of a wall. Rather the photon is absorbed, raising the energy level of an electron, which almost immediatley drops back to its prior - or sometimes lower(?) - energy level. This process, on average, imparts some energy to the mirror, so that its temperature rises. Consequently it will begin to radiate, becoming visible in the infra-red after some finite time.

    But I'm not a physicist, so that might be bollocks.
    The electron always loses that amount of energy it gained.
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  6. #5  
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    I would think it's not possible to have a material which is 100% transparent on one side, while being 100% reflective.

    One-way mirrors are really just a semi-transparent mirror, which lets 50% of light through, and reflect the other 50%, which would happen in both direction. The effect of a one-way mirror is cause by one room being darker then the other.

    http://en.wikipedia.org/wiki/Mirror#One-way_mirror

    However, if there is such a material, I would think your sphere would work as you described.
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  7. #6  
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    Quote Originally Posted by Ophiolite
    Never apologise for lack of knowledge. Only apologise for claiming more knowledge than you posses. This you have not done.

    On the face of it the obvious answer, the one you have come up with, appears correct. It may be, but I am always suspicious of the obvious answer. Here is another possibility to consider:
    If we are admiting photons to the interior of the shell, then we are increasing its internal 'store' of energy. My understanding is that photons do not reflect off of a mirror in the same way a ball bounces of a wall. Rather the photon is absorbed, raising the energy level of an electron, which almost immediatley drops back to its prior - or sometimes lower(?) - energy level. This process, on average, imparts some energy to the mirror, so that its temperature rises. Consequently it will begin to radiate, becoming visible in the infra-red after some finite time.

    But I'm not a physicist, so that might be bollocks.
    This seems like the most plausible answer to me. Even with this answer, all the human eye would see is black, until the heat from the reflection became so great that the radiation became visible. But that would take a while.
    I demand that my name may or may not be vroomfondel!
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  8. #7  
    Forum Ph.D. william's Avatar
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    Hi mikesnail,
    You are correct. At first you shouldn't see anything. But as time grows, so does the energy inside the sphere. Eventually it will radiate as a blackbody. As a matter of fact, this sort of scenario is how a lot of physics professors introduce blackbody radiation to students for the first time.

    Cheers,
    william

    (edit): Sheesh, I didn't realize this thread was so old. This forum needs more members I think!!!
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  9. #8 Re: a Black Sphere 
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    Quote Originally Posted by mikesnail
    MImagine having a sphere which is completely 100% transparent on the outside but completely 100% reflective on the inside ( if you chopped the sphere in half, then you would see straight through it from the convex side, whilst if you looked from the concave side you would see a reflection of yourself).

    Mike.
    THis is clearly a trick question EITHER light can pass through the sphere if it is 100% transparent or not, you cannot argue that it is 100% transparent and in the same breath announce light cannot escape. Transparent means that (mostly) light can pass through, it does NOT mean 'enter and not escape'. (you did say 100% after all).

    Anyway if it was 100% transparent (which implies invisibility) you'd pretty soon lose it!
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  10. #9  
    Forum Ph.D. william's Avatar
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    Hi billco,
    I imagine an ideal two-way mirror (spherical I guess). Whether this is achievable in practice is another story....

    Cheers,
    william
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  11. #10  
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    hey thats sum cool sphere i whould like to have one haha
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  12. #11 Re: a Black Sphere 
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    Quote Originally Posted by billco
    Quote Originally Posted by mikesnail
    MImagine having a sphere which is completely 100% transparent on the outside but completely 100% reflective on the inside ( if you chopped the sphere in half, then you would see straight through it from the convex side, whilst if you looked from the concave side you would see a reflection of yourself).

    Mike.
    THis is clearly a trick question EITHER light can pass through the sphere if it is 100% transparent or not, you cannot argue that it is 100% transparent and in the same breath announce light cannot escape. Transparent means that (mostly) light can pass through, it does NOT mean 'enter and not escape'. (you did say 100% after all).

    Anyway if it was 100% transparent (which implies invisibility) you'd pretty soon lose it!
    I agree with billco on this. However we're dealing in theory here so just for ther sake of the discussion lets say there IS a material that is 100% reflective on the inside; in which case Ophiolite must be right.
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  13. #12  
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    it be kinda cool
    keep shining in floruscent light,
    and the temperature will be like the sun after a year hahax
    naw jsut kiddin the metal would melt anyway
    no time for lots of things
    must save time
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