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How to measure refractive index of plastic

  1. Aug 12, 2011 #1
    I have a plastic disk 0.5" in diameter whose radius of refraction is supposedly non-uniform. It will vary by 0.015 at most from the center to the outside.

    I need to come up with some method of measuring the refractive index locally. I have access to:
    helium neon laser
    a photodetector and software
    optical table with moving mounts
    CNC machine shop
    lots of plastic/glass

    Every way I try seems to not work very well. I'll need some ideas from someone experienced please.
     
  2. jcsd
  3. Aug 14, 2011 #2
    Does anyone know if this is too difficult to pull off?
     
  4. Aug 14, 2011 #3

    xts

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    Have you tried to measure Brewster's angle using very narrow laser beam pointing at small spot on your disc?
     
  5. Aug 15, 2011 #4
    I'll try it.
    But the angle doesn't seem to change significantly if the index varies by, say 0.002 which is the precision I desire.
     
  6. Aug 15, 2011 #5

    xts

    User Avatar

    If your disk has well defined thickness of about 1mm (with accuracy of the same order as you require), you may build an interferometer with disc in one arm.

    Other idea:
    If its thickness is about 1mm, such change in refr.index makes it a lens of focal length of about 1m. You may easily measure that.
    Then use reflected light t0 check if both surfaces are really flat.
     
  7. Aug 15, 2011 #6
    Can you tell me a little more about the second idea? I need to find refractive index n as a function of radius n(r). Do I measure the focal length of said "lens" or what? I'm not sure I'm understanding.
     
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