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What causes those reflecton pools on the road on a sunny day?

  1. Aug 9, 2003 #1
    What causes those "reflecton pools" on the road on a sunny day?

    I really hope someone knows what I'm talking about, but on a sunny day (I've even seen it when it was getting dark)if you are "level" (or generaly even)with the road in front of you (not a lot of hills and turns) you can see (if it is straight and flat) a refletionary "pool" in the road. Ofcourse it goes away once you get closer but from a distance (somehow) the tar on the road can reflect things directly over it, and/or if nothing is over it it will seem like a silvery splotch in the road.

    Can anyone explain this? First of all does has anyone also witnessed it? If not get in your car real fast find a relatively flat strait stretch of road and look towards the horizon of the road. You will see what I am talking about.

    Please reply if you have even SEEN it so I don't feel like a lost fool! :smile:
  2. jcsd
  3. Aug 9, 2003 #2


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    On a sunny day the air just above the road becomes heated by the road surface, which is hotter than the environment. This makes this air layer less dense than the air above it, causing a refraction effect in the sunlight.

    Suppose the sun is in the sky roughly ahead of you. A ray of sunlight comes down at the normal angle, but when it encounters the rarer layer it is bent to a much shallower angle and this is what you see. rays apparently coming at you from the road which your mind interpets as highlights off a pool of water.
  4. Aug 9, 2003 #3
    These phenomena can be explained by the facts (1)that light rays undergo refraction, i.e., are bent, in passing between media of different densitites, and (2)that the boundary between two such media acts a mirror for rays of light coming in at certain angles. This is a mirage. Mirages can be photographed. Mirages are what people chase in the desert, for example, when in search of water.
  5. Aug 9, 2003 #4
    marty said: "Mirages can be photographed"

    They can? Cool! So could this "silvery pool" be photographed, and if so how can an electronic device (a didgital camera) interpret the data like our mind does? I thought our mind is what created the illusions once it recieves light. And if an electronic device can pic it up then is it really a mirage?

    Anyway back to the topic. So, you can see reflections of certain things by the refraction angle of the light against the pavement? And it has to be within a certain angle range? Because that would explain why it dissapates as you get closer.

    Thnx for your time.
  6. Aug 9, 2003 #5


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    This is a phenomenon that I was thinking might be used for millitary purposes. If a ship at sea could release gasses (maybe hot air and steam from the boiler?) with enough controll over temperature and humidity as to create this effect artificially, perhaps they could saround themselves with a pocket of air that produces a mirage. This would create the illusion of a puddle of water sitting on the surface of the ocean, and might allow ships that are stealthy to radar and sonar to become nearly invisible to the eye as well.

    Have to be carefull, though; defelecting EM waves around a metal object sitting in salt water within an envelope of warm humid air MIGHT set up a magnetic field that would cause the ship to phase out of our continuum, leaving people half-sunk into the deckplates or vanished without a trace...
  7. Aug 9, 2003 #6

    Wearing out that VHS tape of a certain movie I see. :wink:
  8. Aug 9, 2003 #7
    Yes. The process is called 'total reflection', and works only for incident angles larger (wrt. to the vertical) than a certain limit which can be calculated from the ratio of refractive indices (in this case dense air/thin air).

    This also means that the rays in question come from the sky rather than the sun, that's why the patches appear blueish-white. And yes, I've often seen it, but never photographed it. Should work, though.
    Last edited: Aug 9, 2003
  9. Aug 10, 2003 #8

    Chi Meson

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    If you can see it (and it's not a halucination), you can photograph it. Visible light obeys the same laws as they travel through the lens in your camera as when they travel through the lens in your eye.

    By halucination, I am referring to all tricks of the mind that occur under many varied circumstances. A "mirage" that two people can see is no such trick, but actual photons of light that have had their paths altered by the well-known physical phenomena of refraction and reflection.

    Lots of science fiction writers and fans of phantasy will disagree with me, but if they do, then they are in the wrong forum.
    Last edited: Aug 10, 2003
  10. Aug 10, 2003 #9
  11. Aug 10, 2003 #10
    Wow, arcnets! That's exactly what I was talking about. I'm glad to know that I wasn't just going crazy.:smile:
  12. Aug 10, 2003 #11


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    Next time you see a mirage on the highway, look at it closely. Frequently you can see the reflection of trees or telephone poles as well as the sky. When you get a chance ot observe these other things reflected then it becomes obvious that the mirage is but a mirror on the road.

    I have also made a very strong sugar solution in a straight sided clear bowl. When such a solution is left undistrubed it forms a density gradient, with a higer concentration of sugar near the bottom of the dish. I then pointed a laser into the side of dish. The beam was bent till it reflected off the bottom of the dish and then was again bent back to its original path. The intire path of the beam was visible due to dispresion from the particles in the solution. The gradient in the sugar solution behaved in the same manner as the density gradient of hot air near the roads surface.
  13. Aug 11, 2003 #12
    Those reflections are also what have led mant a deser traveler to their doom. I wonder if the heat could take the reflection we see and add other halucinations to it to make it seem more believable.
  14. Aug 11, 2003 #13


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    It's not just the heat. In general, people often see what they want to see - it is programmed as part of our brains. So I agree, you brain may well act in misinterpreting confusing and perhaps contradictory information.
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