Experiment 55: Fresnel Lens Solar Death Ray!

A very long time ago, one of my friends introduced me to The King of Random's videos on YouTube.  I was impressed, and it wasn't long before I found his video on making a solar death ray using a Fresnel lens from a rear projection TV and some cheap wood boards.  I had to try it!

After finding a rear projection TV on craigslist for free and just barely managing to compress it into a minivan for transport, I began searching for its Fresnel lens.  The TV was absolutely enormous, and it was really heavy as well.  I eventually found out how to take off the lens and carefully set it aside so it wouldn't get scratched.  Other useful things I found in the TV were castor wheels (used in my ball mill), mirrors (CO2 laser, perhaps?), and various circuit boards and lenses that I haven't used yet.

The Fresnel lens itself is quite floppy, and it is also large and unwieldy, which isn't good.  The King of Random's video showed how to build a nice frame for the lens that held it rigidly but still allowed for easy rotation to follow the sun.  I followed his instructions closely, and the frame turned out to be inexpensive and very helpful.  I also used this Instructable to find the lens focal length so I knew where to put objects for best burning performance.  My death ray's focal length is about 35".  I put a board across the two cross-pieces on the frame so that objects can rest there at a fixed distance from the lens (instead of me holding them in midair).  Finally, I scratched each lens side with my fingernail to find out which side had grooves on it.  The grooves should face the sun for best performance.  In use, I also try to line the lens up so that it is perpendicular to the sun's rays that are hitting it.

Concentrating over a square yard of sunlight into a square inch makes a very bright spot of light, which can be problematic for unprotected eyeballs.  I bought some #10 shade cheap-o welding goggles from Harbor Freight and use them every time I play with the death ray.  It is impossible to see what is happening otherwise, although my iPad can see details inside the bright spot.

I love using my Fresnel lens.  It creates a searing-hot spot of intensely focused sunlight, capable of melting or burning many things.  Over the years, I have melted over a quarter's worth of zinc pennies using this Fresnel lens.  Yes, melting metal with sunlight!  While this lens cannot melt iron or rocks as some videos show, I have melted and compressed HDPE milk jug shreds into a usable billet by placing them in a soup can "oven" under the death ray spot.  My Fresnel lens has even burnt all the way through inch-thick wood boards!  This death ray cost less than $20 to make and uses freely available energy to do incredibly destructive things, which is exactly why I love it!