I found a company that sells giant fresnel lenses. What is a Fresnel lens? Think of a lighthouse or the top of an overhead projector. As light goes through the lens, spiral cut ridges focus the light to a focal point. You end up with the equivalent of a giant magnifying glass or parabolic mirror. There is some really juicy math invovled in calculating how to create a lens, determining the focal length and lens strength.
I framed the lens with some 2“x2“ lumber and secured it with electrical tape. When I carried the lens out of the garage, I knew we were in for a good show. The asphalt driveway started smoking. Our first target was a penny which the lens quickly melted into slag. Next up was a hamburger. We widened the focus and it flash cooked the top of the burger.
What I learned:
- You really need welding goggles. I have none so we had to use a spotter from a distance to help us focus. If you knelt down and tried to focus it without eye protection, you would surely suffer eye damage.
- If the sun is directly overhead, the lens is easier to focus and burns significantly hotter.
- Burgers cooked with a giant death ray don't taste very good. It's about like cooking in a microwave: you don't get the smoky taste that a BBQ gives you.