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Refractive index of asphalt?

  1. Sep 22, 2014 #1
    Asphalt in roads sometimes appears very black through polarizing glasses. I imagine it is possible to measure the angle of reflection with the highest degree of polarization, θ (Brewster's angle), for asphalt. For transparent materials, the refractive index is related to θ, n = tan θ. Is the refractive index a property that exists for nontransparent materials like asphalt as well?
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  3. Sep 22, 2014 #2

    Andy Resnick

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    I'm a little confused by your question: first, asphalt is not smooth (there is diffuse scattering), and second, asphalt is absorbing. Plus there are lots of variations in 'asphalt', especially concerning weathering.

    What are you really asking?
  4. Sep 22, 2014 #3
    Ok. A common observation: the sun, from the south, is shining on an asphalt road. An observer is watching the road from the north. By rotating his polarizing glasses, he observes that the light reflected by the road is polarized (works best if it is new, dark asphalt). However, light reflected by the pavement next to the asphalt is not polarized.

    Question: why is the light reflected by the asphalt polarized? Could it be the same mechanism as why light reflected by glass under certain angles is polarized?
  5. Sep 22, 2014 #4
    What you're observing isn't refraction from the asphalt, but from the hot air layer directly OVER the asphalt. It is the refractive index of the air that you want.

  6. Sep 22, 2014 #5
    Are you sure? Could it be that the surface reflects light polarized in one direction, while light polarized a different direction is absorbed? Could reflection off shiny asphalt be polarized?

    Would this be easy to test? Does it work on cool asphalt? Or is it only asphalt that's almost melty? If you could arrange for a fan to blow cool air on hot asphalt, would that make the polarization go away?

    If you can see the polarization reliably enough to be sure when it's there, or better yet measure it, this might at minimum make a great high school science fair project.
  7. Sep 23, 2014 #6
    No, what you are referring to is called a mirage or fata morgana. A mirage doesn't look like asphalt but like a distinct puddle of water on the road near the horizon. A mirage reflects the sky without changing the polarization of the light.

    Reflection at a surface doesn't care about absorption below the surface. Reflection on black glass is the same as reflection on transparent glass.

    It is easy to observe, and it works on cool asphalt. Many people will remember having observed it already. Go sit in a car wearing polarizing glasses and look through the rear window at the road behind the car (the driver could see it via the rear view mirror). The rear windshield (tempered glass) is a chessboard pattern of oppositely rotating polarizers. If the car is driving away from the sun you will see the chessboard pattern, because the light from the road is polarized (if it is asphalt). If the car is driving in the opposite direction, you won't see the pattern, because the light from the road is unpolarized.
    Last edited: Sep 23, 2014
  8. Sep 23, 2014 #7
    If it's all understood then I guess it wouldn't make a good science fair project after all.

    So your original question was about angle of refraction? I doubt that is important when all the refracted light is absorbed. There might be some circumstance where it's useful anyway, but I can't think of one.

    Maybe if you could somehow cut off a layer of asphalt that's thin enough, some light could get through and you could measure the angle of refraction and see whether it fit the prediction.

    Similarly with gold, some green light gets transmitted through gold leaf, you could check it's angle of refraction.
  9. Sep 23, 2014 #8
    I don't know if it is well understood, I just repeated my interpretation. I am not getting confirmation from anyone here. Because of the confusion I restated it with less care for doubt. But I am still wondering if the reflection of light on an asphalt surface occurs due to the same mechanism as reflection on glass. And whether asphalt has an index of refraction, even though it isn't transparent.
  10. Sep 23, 2014 #9
    You didn't give a lot of detail, and I wondered whether the automobile back window polarization might come from some other source of polarized light, like the sky.

    Anyway, I certainly think it's plausible that smooth asphalt would reflect polarized light just like other smooth surfaces. And it makes sense that the reflected light would be polarized independent of whether the other light is transmitted or absorbed.

    An opaque material could have an index of refraction just as a poor man could have an optimal ratio of stocks versus bonds for his investments, if he had money to invest.

    Opacity isn't usually an absolute, even transparent materials absorb some, and if it's a uniform material you'd expect the absorption to be a logarithmic process -- some thickness will absorb half the light, double the thickness absorbs 3/4, triple that thickness absorbs 7/8, etc. Opaque materials just need a thin layer to absorb half.

    Sorry I don't have anything more useful.
  11. Sep 23, 2014 #10


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  12. Sep 23, 2014 #11
    Thank you!

  13. Sep 24, 2014 #12


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    That's not true in general but only for weakly absorbing substances. Try to write with a coloured marker on a black sheet of paper. You will see that the colour of the reflected light is the same as the colour of the light the ink absorbs. The general formula for reflection depends on both the real and imaginary part of the refractive index alike.
  14. Sep 28, 2014 #13
    Thanks. So I could expect the highest degree of polarization when looking at asphalt at 32° below the horizon, while the sun's height is 32°. The Brewster angle (58°) is about the same as for glass and water.

    The chessboard pattern is within the borders of the object emitting polarized light, which is usually the asphalt road.

    Last edited: Sep 28, 2014
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