Solar Death Ray
The Solar Death Ray is a giant magnifying glass, fun for starting things on fire or melting them.
To be technical, it’s a Fresnel lens, named after the inventor, Mr. Fesnel, a cool old French scientist. Fresnel lenses were originally used in lighthouses, and are still with us today, commonly used for projection. You might remember them from grade-school with those overhead projectors.
Another great more recent use was for the fronts of Projection Televisions. That was the technology for big-screen TVs which immediately preceeded flat-screen LCD and Plasma monitors. Inside the projection TV were three projectors (red, green, and blue) which would shine on a mirror and then the large front fresnel lens.
One day, I was at the Electronics Recycling Center, dropping off a few items. Right in front was a projection TV in fairly good overall condition. I noticed that the lens was held on by only a simple piece of trim. Removing two screws would allow me to pull it right out. Since I always have my Leatherman multi-tool on my, I used the Philips screwdriver to remove the two screws, pull out the lens, and take it home with me.
The lens itself is only 1/8″ thick plastic. It’s bendable to the point of being floppy. I’d need some sort of a frame to hold it flat. A table-saw blade is typically 1/8″ thick. By simply running some 2×4″ boards through the table-saw, with the blade set low, I could cut a groove in the wood which would perfectly hold the plastic lens. I cut the boards to length to go around the lens, and held them in place with a few wood screws.
To mount the frame, I built a yoke from some more 2x4s. Two short pieces of pipe were screwed to the frame and then went through holes I drilled in the yoke to allow the lens to tilt up and down to match the height of the sun in the sky.
Lastly, I screwed a piece of pipe to the bottom of the yoke to match a base I already had. The pipe could rotate in the base to track the sun east to west as it moves through the day.
I call the finished project the Solar Death Ray!
It’s a lot of fun simply to try to heat, burn, or melt various items. It’s a great little backyard science experiment, and kids love it!
The focal point of the lens is about 30 inches away, and the light is so bright that one needs eye protection. I typically wear my welding helmet. I’ve also experimented with making pin-hole lens sunglasses, which have worked well too. The lens itself is 36″ wide by 48″ tall with a 60″ diagonal measurement.
HOW HOT?
So, how much heat CAN we get from the solar death ray? I don’t have an appropriate thermometer to stick in front of it, but I do know the melting point of various metals!
- Zinc – 787℉
- Aluminum – 1221℉
- Copper – 1984℉
I can easily melt zinc with the Solar Death Ray. Aluminum melts, it takes a little more work and time and I need to make sure the ray is properly focused. I can NOT melt copper. When trying to melt modern U.S. Pennies, the zinc completely melts inside the copper coating. The copper itself becomes very soft, but doesn’t actually melt. Based on this, I know that the Solar Death Ray can create heat greater than 1221℉ but less than 1984℉.
I’ve played around a bit making videos burning and melting things with the Solar Death Ray. I hope you enjoy them! The following is a playlist of a number of videos.
Learn more about lenses at Wikipedia: https://en.wikipedia.org/wiki/Fresnel_lens
