Fun with a big lens


The photo below shows a Fresnel lens boiling water in a frying pan on my driveway. The pan is set well above the focal length, so it won't melt. The size of the spot of light is just a bit smaller than the frying pan, about 6 inches across.

Solar furnace

The frame is made of 1 inch by 4 inch lumber, supported by 2 inch by four inch common studs. The lens itself is 40 inches across, and 30 inches high. It came from a 40 inch projection television set.

Below is a photo of four U.S. pennies that were placed at the actual focus, a spot of light about the size of the hole in the penny (1/4 inch across).

Melted pennies

Three of the pennies were of the old copper alloy type (pre-1982). They contain 95% copper and 5% zinc. The penny on top was the new copper plated zinc type. It is 97.6% zinc (all in the center) and 2.4% copper (all in the plating). Zinc has a lower melting point than brass. Much of the zinc actually burned away, leaving the pitted surface you can see in the photo. The copper plating melted and dissolved into the zinc, making the bright gold colored brass lump that joins the other pennies together. I then moved the focus to the bottom penny, and melted the hole in it. The entire procedure took only three or four seconds.