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Could be a daft question but needs asking

  1. Nov 13, 2006 #1
    I was trying to demontrate to some GCSE student total internal reflection, when one of the students pointed out that what I was calling total internal reflection was not really total because there was actually still some light escaping from the plastic prism we were using.

    Was this just due to the fact that the prism was a rather grotty perpex affair? I changed to a very nice brand new glass one, then a parallel sided block and then a semi-circular one, all the time the said smidgen of light still plainly visible even though the incident ray of light was well beyond the critical angle and there was no emergent refracted ray only the so-called totally internally reflected one.

    Can any one explain the above? Is it just the case the even with my nice new glass prisms they are not really optically flat or is there something else at play here?


  2. jcsd
  3. Nov 13, 2006 #2
    There's no "total" anything when it comes to reflection or refraction, etc. Some light will escape as the students observed. Some light will also be absorbed by the material too. Even in a plane mirror, some light is being absorbed and even refracted through the mirror.
  4. Nov 13, 2006 #3


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    How were you detecting the emergent ray?
  5. Nov 13, 2006 #4


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    It looks like he's referring to the faint spot that you can see on the face of the prism itself, where the beam is "totally" reflected inside the prism. I think this is due to microscopic irregularities in the surface of the prism.
  6. Nov 13, 2006 #5

    Claude Bile

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    When you get total internal reflection, the beam actually penetrates through the glass (or perspex or whatever)/air interface slightly, this is what's called an evanescent wave, or evanescent field. The evanescent field drops of in intensity exponentially as you move away from the surface. Typically an evanescent field will at most penetrate about 1 micron through the interface. **DO NOT CONFUSE THE EVANESCENT FIELD WITH A REFRACTED BEAM** Think of the totally internally reflected beam going through the boundary slightly before being reflected back.

    What this means is that light will actually scatter off the surface, dust that happens to be on the surface, even the surrounding air molecules, because the beam is not wholly contained within the solid medium. This is where the observed 'smidgen' of light is coming from.

    Interestingly, this is one reason why optic fibres have a cladding layer, rather than just a naked core.

    Last edited: Nov 13, 2006
  7. Nov 14, 2006 #6
    These are interesting responces. The thing that was bothering me about this is that I'd been taught that we use prisms in situations where the most accurate reflection is required and the the efficiency of prisms is something like 99% as opposed to something like 95% for regular mirrors. Whatever I'm guessing at these numbers but it seemed to me that with that much light 'escaping', how could prisms be that effiicient for the purposes of reflection?

    Claude Bile's answer about evanescent fields is very very interesting and I would like to persue that line of enquiry further. So thank you very much.


  8. Nov 14, 2006 #7

    Claude Bile

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    Here is a neat little discussion on evanescent fields you might find interesting.

    http://www.physics.harvard.edu/~tomhunt/pubs/evanescent.pdf [Broken]

    Last edited by a moderator: May 2, 2017
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