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I noticed when riding my bicycle with some polarized sunglasses that a fresh coat of asphalt appears polarized (ie. when I tilted my head, I saw it as lighter and darker, as when viewing a polarized computer screen with sunglasses on). Fresh seam sealant also appears polarized. Why is this? What property of asphalt causes this? Apparently mayflies have been observed trying to lie eggs on dry asphalt because of this polarization. Here is a study documenting polarization of asphalt.

Apparently lots of dark surfaces are polarized. Perhaps a better question is "Why are dark surfaces often polarized in sunlight?

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  • $\begingroup$ My guess is that fresh asphalt has some liquid component that allows for a little light penetration into it, and so there will be some Brewster's angle-like effect that results in reflected light being at least partially polarized ( en.wikipedia.org/wiki/Brewster%27s_angle ). $\endgroup$
    – user93237
    Aug 3, 2018 at 0:42
  • $\begingroup$ I'm guessing that the act of rolling the paving material creates a surface pattern with the molecules aligned in a single direction, more or less. $\endgroup$
    – Hot Licks
    Aug 3, 2018 at 2:18

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In general, horizontal surfaces reflect light that is polarized in the horizontal direction. The fact that the light is reflecting off of asphalt doesn't change this fact. Due to this, that is why the polarization direction of polarized sunglasses is vertical; that polarization orientation filters out the glare of the light reflected off of the horizontal surfaces in your field of view.

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  • $\begingroup$ This answer comes close. Just add a bit pointing out that fresh asphalt is glossier than old asphalt and explain why smoother surfaces polarize the light more than rough ones. $\endgroup$
    – DanielSank
    Aug 3, 2018 at 9:30
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Polarized light reflects off of all surfaces, old asphalt, new asphalt, concrete or whatever. That’s why they sell polarized glasses for driving. The smoother the surface or the greater the incident angle the more it’s noticed.

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    $\begingroup$ Could you please elaborate a bit more your answer? Why does it reflect off? $\endgroup$
    – FGSUZ
    Aug 2, 2018 at 22:37
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    $\begingroup$ Reasoning by this answer, one would think that the light coming off of a patch of dirt should be polarized too. $\endgroup$
    – DanielSank
    Aug 2, 2018 at 23:06
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    $\begingroup$ @DanielSank yes it’s too bad you down voted because if the patch of dirt is big enough you would notice the reflection of polarized light like the surface of a planet looking at it from outer space. $\endgroup$ Aug 2, 2018 at 23:10
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    $\begingroup$ @Bill Alsept I don't think so. My question is about why fresh asphalt looks polarized. Finding out that some polarized light is reflected off of everything is interesting, but I don't think answers my question. $\endgroup$ Aug 2, 2018 at 23:50
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    $\begingroup$ I think not realizing that polarized light reflects off of all surfaces is a major reason for me asking this question. I just went out and looked at some of the surfaces in that paper with the sunglasses on, and I can definitely notice that not just fresh asphalt, but also older asphalt, and especially water reflecting polarized light. Even the wooden floors of my house were noticeable. It looks like I just never noticed that it was everywhere. $\endgroup$ Aug 3, 2018 at 1:44
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This topic is on polarization of light by asphalt. A interesting video showing the consequence of this phenomenon on insects can be seen in the last half of this 'nature' video: https://www.youtube.com/watch?v=RXrmnpilFoA

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  • $\begingroup$ I am sorry. Seems like this phenomenon was already discussed on this forrum. $\endgroup$
    – Harry
    Oct 7, 2022 at 20:03
  • $\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review $\endgroup$
    – Miyase
    Oct 7, 2022 at 23:24

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