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What is the best way to focus (sun)light using flat mirrors? My goal is to start a fire. Cutting the mirrors is easy.

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Can you bend them? –  AlanSE Jun 1 '11 at 21:53
    
only VERY slightly (they are thin glass and bend like a window pane) –  JoeHobbit Jun 2 '11 at 5:39

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Hi Joe: There are many ways to concentrate solar light with flat mirrors but the easiest way would be to lay them out on a flat plane and only adjust the angles towards the focal point. One could call this a "Fresnel mirror concentrator" like in here:

http://www.findingnew.com/wp-content/uploads/2011/03/solar-thermal-concentrates.jpg

Here is an overview to give you some more inspiration:

http://www.builditsolar.com/Projects/Concentrating/concentrating.htm

Regards, Hans

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From The_Archimedes_Heat_Ray "The 2nd century AD author Lucian wrote that during the Siege of Syracuse (c. 214–212 BC), Archimedes destroyed enemy ships with fire...the device sometimes called the "Archimedes heat ray", was used to focus sunlight onto approaching ships, causing them to catch fire." –  Helder Velez Aug 23 '11 at 12:49

For an example, take a look at this guy who glued 5000 flat mirrors onto a satellite dish to "start a fire", and some more :) Ironically, it was destroyed in an unrelated fire accident later...

Solar Death ray with 5000 flat 1x1 cm mirrors

The R5800 is my latest and greatest solar creation. Made from an ordinary fiberglass satellite dish, it is covered in about 5800 3/8" (~1cm) mirror tiles. When properly aligned, it can generate a spot the size of a dime with an intensity of 5000 times normal daylight. This intensity of light is more than enough to melt steel, vaporize aluminum, boil concrete, turn dirt into lava, and obliterate any organic material in an instant. It stands at 5'9" and is 42" across. Unfortunately, the R5800 was completely destroyed in a storage shed fire on December 14, 2010.

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You need a high degree of concentration to start a fire. With flat mirrors the max concentration factor is equal to the number of mirrors. Is that last statement true? Or can complicated geometries with multiple reflections do better? But in any case, without curvature you don't get true focusing and would need a large number of small flat mirrors.

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You are correct saying the the max concentration factor is equal to the # of mirrors, although I lack a rigorous proof right now. Your claim that a "large number" of mirrors is ambiguous but wrong by most definitions. I would say "a handful" would be sufficient. –  AlanSE Jun 1 '11 at 23:33
    
""I would say "a handful" would be sufficient. "" Are You going to join Jim Grabers "9-fold"? That is nearly two handsful –  Georg Jun 2 '11 at 11:35
    
You might be able to do it with ten or twenty if the mirrors are large, say a meter or two, but you will likely have to wait an hour or so, and hope something has a low ignition temeprature. For smaller mirrors you probably need concentrations of several hundred times. –  Omega Centauri Jun 2 '11 at 15:34

You will want to make the cross section of your device approximate a parabola. This should be relatively easy to do with a large device (say, using 3cm wide strips of mirror to build something the size of an outdoor barbeque); but tougher as you get smaller.

If the parabola is of the form y=(a)(x^2); then the focal length is 1/(4a). By choosing your scaling factors wisely, you can control whether the focal length falls inside or beyond the physical device. The break point between these two regions is picking a=1 and letting x range from -1 to 1.

And, as has been pointed out in another post, the best effective sunlight magnification you get will be 'z', where 'z' is the number of flat mirrors. If you were to do something like mold aluminun flashing to a parabolic surface, the theoretically achievable magnification is infinity, and the practical magnification could easily be in the hundreds.

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Googling around, I find that wood can ignite when struck by 4.3 kiloWatts per square meter, and that the solar constant is 1.3 kiloWatts per square meter. This implies 4 confocal solar images would be good enough to start a fire under ideal conditions. I bet it takes more than that, but maybe not too many more. Good luck and be careful.

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I have seen fires started with a small handheld 3X lens. That implies you don't need more than 9X concentration. So you should probably be able to do it with ten mirrors or less. –  Jim Graber Jun 1 '11 at 23:23
    
""with a small handheld 3X lens. That implies you don't need more than 9X concentration."" ROFL! I'd recommend You read about the meaning of that "X-fold" of lenses. –  Georg Jun 2 '11 at 10:28
    
""I find that wood can ignite when struck by 4.3 kiloWatts per square meter, "" Another nonsense. To start a fire, You need temperature, not some power/area! And temperature in such cases is a function of power fed (to a piece of wood) minus power lost (eg by convection from that piece). Without that "cooling" factors any power/area is useless. –  Georg Jun 2 '11 at 10:38
    
If you google "lens fire starter", you will find lenses of 3-5 power sold for that purpose. They are only an inch or two in diameter. Boy scouts use them all the time and they work. –  Jim Graber Jun 5 '11 at 12:04
    
marioloureiro.net/ciencia/ignicao_vegt/wood_ign.pdf Here is the reference on igniting wood which gives the above cited figure. –  Jim Graber Jun 5 '11 at 12:06

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