Focusing Light with Flat Mirrors What is the best way to focus (sun)light using flat mirrors? My goal is to start a fire. Cutting the mirrors is easy.
 A: 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.

A: 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
A: 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.
A: 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.  
A: 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. 
