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Producing heat(fire) using magnifying Glass

  1. Aug 18, 2012 #1
    Hello! :D I wish to know how is Magnification related to Focal Length or Focal point in terms of magnifying lenses? We are researching how magnifications affect the heat produced by magnifying lenses, unfortunately, I found out that it's mostly the size. does this mean that the magnification really has no effect on the heat produced? or What other factors affect it?
     
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  3. Aug 18, 2012 #2
    When you hold a magnifying glass in the sun and place something at its focal point, it creates a high temperature because it takes all of the radiation that hits the large area of the magnifying glass, and focuses it into a tiny point, making a high concentration of heat. You can focus light at the focal point of any magnifying glass, but the larger the area of radiation you can focus onto the point (the larger the magnifying glass), the higher the concentration of heat at the focal point.

    The maximum angular magnification (MA) of a magnifying glass is given (approximately) by:
    [itex]MA=\frac{25 cm}{f}[/itex]
    f is the focal length, meaning that higher magnification is achieved by a shorter focal length.
    A magnifying glass with a high magnification will mean that the focal point is closer to the glass, but this does not effect the concentration of heat at the focal point.
     
  4. Aug 18, 2012 #3

    HallsofIvy

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    The point is that there is NO "magnification" involved here at all. It is simply a matter of focusing all the light that passes through the glass on a single point. And how much that is depends on the area (and so radius) of the glass.
     
  5. Aug 18, 2012 #4

    sophiecentaur

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    The 'magnification factor' of a convex lens relates to how close it can put a virtual image to your eye that you can still focus on. It relies on your own individual powers of accommodation. (That will be where the "25cm" in the above post comes from) This is not really relevant for a burning glass. The main factor in a burning glass is the actual area of the lens (the energy gathering potential) The temperature that you can obtain at 'the spot' will depend, among other things, on the size of spot you can produce.

    If you look at the size of the Sun's image on a camera sensor, for a wide angle lens, you can see it will be a lot smaller than for a telephoto lens. The 'f' number for the aperture of a lens gives you a clue about the suitablilty for burning because any lens with a given f setting (e.g. f4) will have the same intensity of light per unit area of the sensor. Now, the f number is the ratio of focal length to aperture - so a long lens will need a wider aperture (in mm) for the same intensity BUT, of course, the Sun's image will be bigger so more total energy will hit the sensor.
    Which brings us to the "among other things". If you have a highly conductive target then the maximum temperature reached will be limited by the total energy flux (lens area). But if you are trying to heat a really good insulator, the maximum local temperature reached will be limited by the f number (i.e. the intensity of the spot - whatever size it is).

    So the answer must be ---- It Depends! Sorry (But a good big-un will always beat a good little-un)
     
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