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Why is the birefringence pattern in a car colorless?

  1. Oct 15, 2015 #1
    When looking through polarizing glasses at the rear window of a car (tempered glass), a black and white pattern appears. Supposedly, this is the same kind of birefringence that causes colored fringes in a plastic sheet, seen through a polarizer. It is clear that retardation and interference in the plastic sheet results in colored fringes. What is so different about the rear window of a car that prevents the colors?

    Lcd display seen through rear window and polarizer
  2. jcsd
  3. Oct 15, 2015 #2

    Andy Resnick

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    Offhand, I would say it's because sunlight has a broad spectrum- the colors are 'smeared' out.
  4. Oct 15, 2015 #3
    For comparison, a photo of a birefrigent cd case, seen through the same polarizer, and looking at the same polarized lightsource (an lcd display) in the background. Color appears because birefringence means retardation. Why is a birefringent rear window different?

  5. Oct 16, 2015 #4

    Andy Resnick

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    Good point.

    Car windows are sometimes laminated glass, not a single sheet of plastic. There can be multiple layers of plastic, glass, and adhesive films present, and no doubt this has an effect on the photoelastic properties of the window. Even 'simple' tempered glass can have complex photoelastic properties:


    The second reference mentions that the characteristic pattern observed through car windows may not be correlated with internal stress.
  6. Oct 16, 2015 #5
    My question was about the rear window, which is unlaminated, without an adhesive film. It was not about the laminated front window because the black and white pattern doesn't appear there.

    The old prototype of tempered glass, Prince Rupert's drop (click), apparently induces colors between crossed polarizers. Maybe tempered glass has a black and white or 'zebraic' extinction pattern if the thickness of the glass is about λ n/Δn, and do colors appear at greater thickness. I also noticed that polycarbonate cd disks induce a black and white 'Maltese' cross which is typical for uniaxial birefringent materials. Maybe somehow that's another explanation.

    Rupert's drop between crossed polarizers; image from the video by Smarter Every Day at 3m20s

    Clear substrate cd disk between crossed polarizers: Maltese cross
    Last edited: Oct 16, 2015
  7. Oct 16, 2015 #6

    Andy Resnick

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    Well, now you've got me curious..... this is more interesting than I thought.
  8. Oct 17, 2015 #7


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    If the sample is rotated 90 degrees does the light-dark areas change?
    You might have to rotate the LCD screen and polarizer to get the rotation wrt the sample correctly done.

    With the colored sample, rotate that one and the colored areas should change.

    The usual explanation for the back window for reflected light is that the window changes the light to horizontal which your polarized sunglasses block out. The spotted areas reflect vertical light which your sunglasses do not block. Not frequency dependent it seems. Not really sure about vertical polarization. If one tilts their head sideways the polarized sunglasses would see black where the light areas are now. Does that happen? I have never tried.

    Still curious.
  9. Oct 17, 2015 #8

    Andy Resnick

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  10. Oct 17, 2015 #9
    I agree that the absence of colors means that the retardation is less than 1/4 λ.

    90° rotation of the crossed polarizers (together) results in an identical black and white pattern.

    Reflected light is slightly more complicated than transmitted light. For transmitted light, the black lines are the areas where the rear window does not change the polarization direction of the incoming light. The extinction is caused by the crossed polarizers.

    Apparently the black curvy lines are the so called isogyres. When rotating the crossed polarizers over 90° (together), it would turn out that each point of the window has its own polarizer angle α between 0° and 90° that makes it black. That is the privileged direction in that point. The perpendicular direction is privileged as well. The pair of privileged directions could be marked by drawing a small cross in each point of the window. By connecting the crosses the pair of privileged directions could be visualised as two lines systems on the window.

    The two privileged directions are actually the slow polarization direction and the fast polarization direction in each point of the window. If the crossed polarizer direction is parallel to either, the extinction will be maintained, and that point will be black.
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