DentonDoc wrote on Jan 30^th, 2011 at 9:33pm:

When I put my pocket survival sleeve into the ditch-kit (survival cards, compass, etc.) it also includes a small fresnel lens ... its in the photo (above) but since its clear, its hard to see. My original thought for the use of the lens was to use its magnifying properties to look at small items (map, if I loose my glasses; close-up to get a splinter out of my finger, etc.).  However, it dawned on me that it is a magnifying "glass."  As a kid, I played around with a glass to burn a few small things, but never really concentrated on the process to the extent that I could make a fire.  As it turns out, you CAN make fire with a magnifying glass (including a fresnel lens).  It helps if you have a bit of char cloth or piece of paper, but you can ultimately coax the ember produced into flame.  (It also can ignite a match, if you should loose the striker.) Then I began to wonder about broader applications of a fresnel lens.  Since it produces heat, can you boil water with it?  Well I did find several youtube videos of boiling water and actually cooking with a fresnel lens, but the setup seemed to involve one that is roughly 3 X 4 feet.  Nope, I'm not packing something that size; but I do have a fresnel lens that is roughly page size (6 X 10 inches).  Now, that's packable!   Anyone have any experience with using a fresnel to heat/boil water.  It could be useful for cooking, but I'm also thinking about potentially purifying water.  I started a small-scale trial, but stopped when it started to melt the plastic of the thermometer I was using.

Follow-up (on the 6 X 10" lens):

Nice sunny day outside today ... mid-60's.  Thought I'd get back to playing again (and leave you guys alone for a while ... no luck there!)

I haven't tried to boil water yet with the larger lens, but that could be a little bit of a problem.  I had previously (partly cloudy day, indoors) tested the focal point of this lens.  I succeeded in melting a nice arc on a dark plastic bottle.  (Yeah, the sun does make an arc in the sky this time of year).  So to reach a sustained boil, you WILL need to re-position the lens position ever so often.

So, today I thought I'd get back to some basic fire making ... larger lens, no char cloth.  I took a small piece of t-shirt (cotton), some dry grass, a couple of leaves and a small piece of birch bark to catch the flame when it started).  Once the focal length was set correctly (about 2 1/2 feet with this larger lens), the material started to smoke almost immediately.  But I gave it a good 30 seconds before I began to add more oxygen.  Within a minute I had a flame.  

Who says science can't be fun!  :)

dd

BTW:  My bum wheel has just about mended, so I'll be back to walking soon and will leave you poor folks alone!  ;)