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Is birefringence different in plastics, glass and calcite?

  1. Jun 5, 2013 #1
    1) Is birefringence different in plastics, tempered glass and calcite crystal?

    In plastics I see rainbow color bands (when looking through polaroid glasses), in tempered glass a colorless checkerboard pattern (again viewed through polaroid glasses), and in calcite double images. These different birefringence phenomena don't seem to occur together in materials, but I even could not find birefringence category names to distinguish them.

    2) Is there a formula for the displacement of the extraordinary image relative to the ordinary image, in a calcite crystal?
     
    Last edited: Jun 5, 2013
  2. jcsd
  3. Jun 5, 2013 #2

    davenn

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    Yes I would expect it to be very different.
    After all its different even between crystals of different minerals, say calcite, silia, mica
    its one way to be able to distinguish different minerals when doing optical mineralogy ( Petrology)

    here's an example of the mineral Epidote
    attachment.php?attachmentid=59318&stc=1&d=1370477764.jpg

    using birefringence, exticntion angle etc, determination of individual minerals within the sample can be done

    When I was doing my geology degree, this was one area I really enjoyed. Grinding thin sections of minerals on microscope slides. Then doing the analysis of them with the microscope.


    regards
    Dave
     

    Attached Files:

  4. Jun 6, 2013 #3
    In Wikipedia, this photo of an Airbus cockpit window is used as an illustration of stress induced birefringence. The color contours are assumed to correspond to tension lines because the left and right window colors are mirror images. However, the photo was taken without a polarizing filter on the camera, so there is some doubt that birefringence is involved here. What else could explain these color contours?

    640px-LHcockpitWindow.jpg
     
  5. Jun 6, 2013 #4
    The colors may be due to thin layer interference.
    Maybe in the inner layer of polymer.
    The tension in the window may change the thickness of the layer a little so the colors may indeed be a map of the stress. Only the mechanism is a little different.

    A similar effect can be seen when you press together two microscope slides. The pattern of interference colors changes when you squeeze the "sandwich" with your fingers. But in this case there is just air between the slides. I don't know what is the airplane windshield structure but is very likely to be a multiple layer structure.
     
  6. Jun 7, 2013 #5
    Isn't there a formula for the displacement of the extraordinary image relative to the ordinary image, in a calcite crystal?
     
  7. Jun 8, 2013 #6
    "The phenomenon of double refraction is based on the laws of electromagnetism, first proposed by British mathematician James Clerk Maxwell in the 1860s. His elaborate series of equations demonstrate that the velocity of light through a material equals the speed of light in a vacuum (c) divided by the product of the square root of the material's dielectric constant (e) multiplied by the magnetic permeability (m) of the medium. In general, biological and related materials have a magnetic permeability very near 1.0, as do many conducting and non-conducting specimens of interest to the microscopist. The dielectric constant of a material is therefore related to the refractive index through a simple equation:
    ε = n2
    where e is a variable representing the dielectric constant, and n is the material's measured refractive index. This equation was derived for specific frequencies of light and ignores dispersion of polychromatic light as it passes through the material. Anisotropic crystals are composed of complex molecular and atomic lattice orientations that have varying electrical properties depending upon the direction from which they are being probed. As a result, the refractive index also varies with direction when light passes through an anisotropic crystal, giving rise to direction-specific trajectories and velocities."

    http://www.microscopyu.com/articles/polarized/birefringenceintro.html
     
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