I was looking for a Christmas present for a farm labourer who has a special place in my life. I decided on an inexpensive plastic Fresnel lens, about the size of an A4 / Letter sheet. Among other things, this would be useful for lighting a fire, by focusing the rays of the sun. I tried it out today. It didn't merely make smoke -- it lit a fire with an explosive "BOOF!" I was impressed. Thanks to the US Lighthouse Society for the photo of a Fresnel lens.
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