A normal lens this big would be several feet thick and weigh a proverbial ton (the right-hand image below). However, it's possible to remove much of the inside of a lens and collapse down the shape without introducing too much distortion (the left-hand image):
Fresnel (pronounced "freh-NELL") lenses are used in overhead projectors and lighthouses. We've been using ours, however, to see what happens when you focus 1,000 watts of sunlight onto a single point. It's like when you were a kid and tried to burn ants with a pocket magnifying glass — but 400 times stronger. We built a wooden frame to keep the lens flat and focused, and a stand to hold it steady:
The light in the focal point is so bright that you can't look directly at it without welding goggles.The lens maker claimed you could melt a penny with it, so that was the first thing we tried:
Modern pennies are made of zinc with a copper coating. The bottom row shows what happens when you put a penny in the focal point of the lens: the inside melts away and the coating stays intact (zinc melts at 693 kelvins, copper melts at 1,356 K). But if you heat it just enough, the metals mix and you make brass (the gold-colored penny in the middle). Older pennies (those minted before 1982) are almost entirely copper, so they didn't melt (top row).
We also had an aluminum can:
The water we poured in boiled quickly, while the can itself became so brittle that we poked holes through it with nothing more than sunlight.
Then we tried cooking. Popcorn did both what you'd expect and not quite what you'd expect: when you really focus the light on it, it kinda pops but mostly burns. However, if you don't put it directly in the focal point, so the light is spread over a larger area and doesn't heat it up as quickly, you can get a whole bunch of kernels to pop without burning too much.
The steam/smoke coming up from the kernels really highlighted the spectra from the lens beautifully. Our yield was very low (lots of unpopped kernels for each popped one), but at least we had real popcorn!
When we tried to cook bacon, about a third ended up well done, a third was burnt, and a third was uncooked. Cooking with the lens is difficult because it heats stuff up too hot too fast. But the well-cooked parts tasted great, so we added an egg:
(We didn't lens the spoon; we used it to eat the egg afterwards.)
It's been fun experimenting with different lensing techniques and items and we've learned a lot (including where the nearest fire extinguisher is!). These are just the highlights — we've lensed gourds, soap, gummy bears, CDs — you name it. Next on our list: marshmallows!
We've got more details and more pictures of our results on Alan's personal blog. If you have ideas of other things we should try lensing, we'd love to hear suggestions.
