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Immerse a Light Pipe in Water?

  1. Apr 5, 2005 #1
    I'm preparing a demo for my class...

    I have a light pipe (a clear, solid plastic pipe about
    a half-inch in diameter). Incoming light is always
    limited to incident angles less than the critical
    angle -- so there's total internal reflection (the
    basis of fiber optics).

    The pipe is coiled -- like the heating element on top
    of a stove.

    If I immerse the light pipe in a mediam with a higher
    index of refraction than the pipe itself, would the
    pipe's ability to trap the light be affected?
  2. jcsd
  3. Apr 5, 2005 #2
    Yes. The light will no longer be trapped in the pipe.

    In fact, no ciritical angle will exist and the pipe would be unable to
    contain light at any angle.
    Last edited: Apr 5, 2005
  4. Apr 5, 2005 #3


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    Gold Member

    Correct me if I'm wrong, but the pipe will merely act much like a hollow pipe filled with air, right? It'll look the same as if filled with air, reflect and refract the same (though less so because of the smaller diff in refractive index).

    Say if I had some fishtank tubing with an ideally zero thickness wall, and filled it with air, and dunked it, I'd get the same effect.

    So, it's not that the critical angle would case to exist, it would just be flipped, with the refactive index lower *inside* the pipe.

    Oh yeah ... I know what this looks like! When air is trapped underwater, it looks like a mirror. Your fibre optic cable should look like it's chrome-plated.
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