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Water droplets and optics

  1. Feb 18, 2009 #1
    It is said that bush fires in Africa can be started by water droplets in the grass. How could this work? What actually causes the burning?
     
  2. jcsd
  3. Feb 18, 2009 #2

    jtbell

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    Have you ever used a magnifying glass in the sunlight to light fires?
     
  4. Feb 18, 2009 #3
    yes, but i never knew how that worked..
     
  5. Feb 18, 2009 #4

    jtbell

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    The lens (or water drop) concentrates the light that enters it, at its focal point.
     
  6. Feb 19, 2009 #5

    Andy Resnick

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    This is how some old weather-recording devices worked- a large glass ball sat in the center of a ring of paper. If the sun was out, the paper would have a char line, and if it was cloudy, there would be no line.
     
  7. Feb 20, 2009 #6
    OK, and I'm not trying to be silly here at all, but I question the water droplet scenario.

    A water droplet tends to be quite small, and I would imagine also grossly distorts while clinging to, say, a blade of grass or some such thing.

    It seems to me that the droplets size itself, even if it could be shaped into an ideal lens for starting a fire, would simply be inadequate. After all, with a true lens the size of a water droplet, I tend to doubt there is enough surface area to concentrate sufficient sun-light to start a fire. Indeed, could a fire be started with a lens which has a diameter less than that of a pencil eraser?

    But I could be wrong on that aspect.
    Even if I were, a water droplet in nature is so grossly deformed apart from an ideal lens that the potential is even much less, it seems to me.

    Just my thoughts...
     
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