A Fresnel lens looks like a piece of glass or plastic with tiny circular lines all over it, one inside the other. It is sort of like bullseyes that are very close together and that you can see through.
It is a type of magnifier. I discovered their existence because of receiving a catalog of optical supplies and another with surplus items in it.
You are probably already aware that you can light things on fire by using a magnifying glass held over it in the sunlight. You focus the sunlight on whatever you want to light and in a few minutes it starts smoldering.
If you use a Fresnel lens, the fire is generally more impressive than using a regular magnifying. The Fresnel lens does a better job of focusing and intensifying the effect of the sunlight to start a fire.
If you want to use a magnifier to start fires as part of your preparedness, it is better to have a Fresnel lens for this purpose.
As is the case with regular magnifiers, Fresnel lenses work better the larger they are. If you have a large enough Fresnel lens, you can melt metal with it. lol. Now I have started something, I am sure.
Most people have never even noticed the existence of Fresnel lenses. They are handy suckers if you like to make things and invent stuff. If you are among the aforementioned group, you really need to get your hands on a Fresnel lens.
I just posted on how to make a cheap solar oven yesterday. I did not mention Fresnel lenses in that post. This is because Fresnel lenses are powerful enough that it would require a lot of care to use on on a cardboard and styrofoam solar oven. Fresnel lenses used on one of those could easily start a fire.
Styrofoam releases dangerous gases when burned and a fire with the aforementioned solar oven would not be a good idea.
I have spent a small amount of time playing with a Fresnel lens, but I fully intend to spend more. Fresnel lenses have great possibilities that have not been explored yet, especially by me. I have plans for those things.