My dad brought one home when I was a kid, I think it was from an old
overhead projector. On sunny days in Colorado Springs (~6300 ft elevation) I
could spot weld coins together with it. And I don't mean sit around for an
hour and wait for things to heat up. I mean put on welding goggles, bring it
into focus on a stack of coins, and almost instant molten pennies. If you
ever see an overhead projector at a thrift store or junkyard, I HIGHLY
recommend getting the lens. It beats frying ants and beetles (VW Content?)
with a little reading glass.

>From: John Carpenter <Trvlr2001@AOL.COM>
>Reply-To: Trvlr2001@AOL.COM
>To: vanagon@GERRY.VANAGON.COM
>Subject: fresnel lens
>Date: Tue, 7 May 2002 18:37:53 EDT
>
>Question
>
>I have a thin piece of plastic mounted on the back window of my RV. It
>magnifies things so I can see better when I am backing up. Why can such a
>thin piece of plastic magnify things? A normal glass magnifying lens would
>have to be curved on both sides and would be much thicker. Answer
>
>
>
>
>If you have ever looked at the lens of a magnifying glass, you know it is
>thick in the middle and tapers to nothing at the edges. In other words it
>is
>shaped like a lentil, which is where the word lens comes from. It would not
>be very easy to make a big magnifying glass lens for your RV because it
>would
>be thick, heavy and hard to mount. The thin piece of plastic you are using
>is
>called a Fresnel lens. It is flat on one side and ridged on the other.
>Fresnel lenses we first used in the 1800's as the lens that focuses the
>beam
>in lighthouse lamps. Plastic Fresnel lenses are used as magnifiers when a
>thin, light lens is needed. The quality of the image is not nearly as good
>as
>that from a continuous glass lens, but in lots of applications (like your
>RV)
>perfect image quality is not necessary. The basic idea behind a Fresnel
>lens
>is simple. Imagine taking a plastic magnifying glass lens and slicing it
>into
>a hundred concentric rings (like the rings of a tree). Each ring is
>slightly
>thinner than the next and focuses the light toward the center. Now take
>each
>ring, modify it so it flat on one side, and make it the same thickness as
>the
>others. To retain the rings' ability to focus the light toward the center,
>the angle of each ring's angled face will be different. Now if you stack
>all
>the rings back together, you have a Fresnel lens. You can make the lens
>extremely large if you like. Large Fresnel lenses are often used as solar
>concentrators.
>
>

Daniel Snow

'82 Vanagon Diesel
'78 Puch Maxi Luxe Moped
'01 Xootr Scooter
http://faculty.haas.berkeley.edu/snow/vanagon/vanagon.html

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