Interesting Stuff

If you missed it, here’s part 1 and part 2 of our adventures with a 4’x3′ fresnel lens. As always, I’ve got a gallery of all the raw pictures, and this is a summary of the good ones.

The same SAFETY NOTES as before apply, with the addition that you shouldn’t eat any food you cooked in the same skillet in which you just cooked non-edible things (plastics, coins, etc). Also, we’ve upgraded our welding goggles, from a shade 5 filter to a shade 12, which is actually good enough that you can stare at stuff in the focal point. It’s not quite good enough to stare directly at the sun, but it’s darn close, and rumor has it that you can watch sunspots through this thing if you don’t look at them too long.

Last time, I left you with a picture of popcorn getting ready to be lensed. Well, it 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! It sounded just like you’re used to, but since it was not in an enclosed container the kernels jumped pretty high as they popped (over a foot, I think?). One even landed in the cup of water we were using to hold down the aluminum foil!

It wasn’t very tasty, mostly because we didn’t cook it in oil/butter or salt, so it was very dry and bland. but it totally (kinda) worked!

A more surprising result was when we tried lensing a grape.

If you watched through the welding goggles, you could see some pretty interesting things. The skin of the grape burned, turned black, split open, and unrolled as it burned, exposing the juicy insides. The water boiled off and made very loud hissing/sizzling sounds. When the water near the surface of the grape had finished boiling away, that part of the grape turned black and charred as well.

It smelled both delicious and terrible at the same time (kinda like burning grapes; no surprise there).

This was so good that we tried kumquats, which turned out to be even better! Like the grape, the skin burned and pulled back to expose the insides. Unlike the grape, the kumquat had lots of small sections to it (somewhat like a citrus fruit). So, one would pop and boil and all this steam would come out, and then as it dried out it would burn through to the next one, which would in turn pop open and start boiling. Some droplets of water were sprayed a whole 2 inches above the kumquat due to the energetic way in which sections would burst open! It continued to boil like this for over a minute. It smelled delicious.

Since we had boiled off most of the juice, it was less than half its original weight. We actually got bored and took it out of the light before it was done boiling, so I think we could have gotten it even lighter. Part of the black char had turned to grey ash, but the char was more interesting than the ash. The kumquat’s skin is mottled, and the burnt part had the same mottled pattern between black and darker black. I don’t know if it was two kinds of char or one kind of char with holes in it.

Also interesting is that we did something to the aluminum foil under the kumquat, in that part had a small hole, and the foil around it was so thin you could see through it!

There’s no way we hit 2792K to boil the aluminum, so we’re not sure where it went.

Since the grapes failed to turn into raisins, we had little hope for tomatoes becoming sun-dried, and indeed they acted similarly to the grapes (the liquid boiled off and the solid burned). I think we’re heating stuff up too fast, and a cooler, longer cooking period is better for foods. We decided not even to bother trying to grill cheese sandwiches, since we’re now fairly sure the bread would burn before the cheese melted.

We tried several kinds of sugary things, all of which burned (no surprises here, but it was fun anyway). Here’s a Frosted Mini Wheat:

and some jelly beans, which melted as they burned:

The Reeses Piece kinda peeled back into layers of candy shell and peanut butter.

We’re pretty sure that most of these sugary things actually produced flames as they burned, but it’s hard to tell because you can’t really look at them.

If nothing else, they smoked.

We also tried Rice Krispies treats in the skillet.

Intriguingly, the top burned pretty badly but the rest appeared untouched.

Perhaps all the air inside them acted as insulation, so the top could get really hot but the rest didn’t heat up much?

As always, remember safety first. Also, if you have more ideas of things we should try, I’m happy to give them a shot.

Edit: check out part 4 for more lensing fun!