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Magnifying Lens & UV

  1. Mar 1, 2015 #1
    Practical question: Will placing a clear UV filter over a magnifying lens inhibit the ability of that lens to focus sunlight to the degree necessary to produce fire?
    I have a window with several "bullseye glass" panels in my home and have noticed that when the bright winter sun passes through at just the right angle, these panels produce a beam hot enough to get the upholstery smoking! Seems like a fire hazard and I'm wondering if applying the UV blocking film sold at home centers will fix the problem.
    Thanks for your insights...
  2. jcsd
  3. Mar 1, 2015 #2

    Quantum Defect

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    I suspect that it will not help. The glass panel probably blocks much of the UV. You would probably have better success with either a neutral density filter or with something that acts like a lens with a negative focal length.

    I believe that I burned paper with polycarbonate lenses when I was a child. Polycarbonate plastic blocks most short wavelength. So the heat must have come from the focused visible light.
    Many of the people at PF were probably such children!:wink:
  4. Mar 1, 2015 #3


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    Experiments and observations are important parts of science. Example of cheap equipment needed to test your idea: a magnifying glass, paper and UV blocking film (or sunglasses with UV protection to try it initially). But try it outdoors on a sunny day :wink:. EDIT: I just saw that Quantum Defect managed to sneak in a reply above just before me :biggrin:.
  5. Mar 1, 2015 #4


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    As has been stated, you have a slight misunderstanding here. UVs are normally blocked by many clear glasses. Ordinary glass windows are often opaque to UV. And so will an ordinary magnifying glass. So you have been focusing on the wrong "culprit" (no pun intended).

    Any type of light, when there's a lot of it and it is focused to a small spot, will increase in its rate of energy density. At some point, such a high energy density rate being absorbed will cause a significant increase in heat. This is what is happening in your case. The easiest solution is simply to put a shade, or some neutral density filters to cut back on the amount of light coming in.

  6. Mar 1, 2015 #5
    Thanks all for the quick and helpful feedback
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