The Optics Of Setting Leaves Aflame

A magnifying glass looks harmless. But combine it with a nice sunny day and you have a weapon of ant destruction and a fire hazard. Even if cheap pyrotechnics isn't your thing, the physics behind how this works is relevant to anyone with eyes.

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IRA FLATOW, host:

Next up, Flora Lichtman is here with our Video Pick of the Week. Hi, Flora.

FLORA LICHTMAN: Hi, Ira.

FLATOW: You've got something special, of course.

LICHTMAN: Of course.

(Soundbite of laughter)

LICHTMAN: This week, we have a home science experiment, but you do need a sunny day. It's fun-in-the-sun science.

FLATOW: Summer science.

LICHTMAN: And I didn't even - I did this a lot as a kid, and I didn't even know that there was some kind of lesson I could learn while I was doing it. So here's what you need: You need the sun and you need a magnifying glass. And I think, probably, most people know where I'm going with this.

If you put your magnifying glass up to the sun, you can concentrate the sun's rays and torture insects, which we do not recommend.

FLATOW: No. No insect was harmed in the making of your video, right?

LICHTMAN: That's true. No insects were harmed, but we did light some dead leaves on fire, so maybe adult supervision too.

FLATOW: I love the smell of leaves when that little - we've all done that, right? You take the magnifying glass, you focus it on some leaves and they catch on fire.

LICHTMAN: Right. It seems like magic, but it turns out that it's optics.

We spoke with Tom Baer at Stanford, and he is a physicist who kindly explained what is going on when you're doing this. And I think, you know, most of us know that you're bending the light to this focal point and that's why it looks more concentrated and so you've got more light in a littler area. You're squeezing more sunshine into a smaller area.

But you can actually calculate, it turns out, how much energy or how much more intense that little spot is than the sunlight that's hitting your hand, for example, with an equation.

FLATOW: Wow. And you show us how to do that on your video. It's our Video Pick of the Week up there in the corner, left-hand side of sciencefriday.com. And you take us through that, and it's fascinating.

LICHTMAN: It was - I was really amazed. It turns out - so we did this with our - so you have to know the diameter of your lens...

FLATOW: Right.

LICHTMAN: ...and you have to know the focal length, which just requires a ruler. But we found out that our magnifying glass was concentrating the light 1,000 times.

FLATOW: A thousand times.

LICHTMAN: A thousand times.

FLATOW: A thousand times what was just be falling on your hand.

LICHTMAN: That's right.

FLATOW: That's why you could burn a leaf with it, right?

LICHTMAN: That's right. So that is a magnifying glass. So you think, wow, a thousand times, that is a lot. But what Tom Baer told us is that, you know, think about your eyeballs, because it, too, has a lens in it that's focusing the light. And I don't know - spoiler alert here?

FLATOW: Go ahead.

LICHTMAN: You should still watch the video.

FLATOW: Hum loudly...

(Soundbite of laughter)

LICHTMAN: It's so cool.

FLATOW: (unintelligible) OK.

LICHTMAN: It turns out that your eye actually concentrates the light 2,500 times. 2,500 times.

FLATOW: Your eye?

LICHTMAN: Your eye.

FLATOW: No wonder we don't want you to look at the sun.

LICHTMAN: This was a good explanation for why you should not look, you know, you see the leaves bursting into flames...

FLATOW: Wow.

LICHTMAN: ...and then you learn that your eye is sort of double that in the intensity of concentration. And that was proof enough for me not to look at the sun.

FLATOW: You're - not even for a little bit, right?

LICHTMAN: No. No.

FLATOW: But you also - a wonderful little aha that you have in the video...

LICHTMAN: Yes.

FLATOW: ...at the end.

LICHTMAN: Yes. At the end, you can see the sun - this was an aha from you, Ira, actually. But if you look through your magnifying glass, you actually are looking at the sun on the paper, right?

FLATOW: Yeah. If you focus, you can actually focus the sun. And if it's going through some dappled leaves like you have it there, you can actually see a beautiful picture of the sun and the leaves and everything...

LICHTMAN: Yeah.

FLATOW: ...right on a piece of paper on the sidewalk even.

LICHTMAN: Yeah. We had a kind of a nice scene-scape because we're on a roof in New York, and so we had some tall buildings and some clouds going across the sun. But it was pretty cool. I didn't know that this...

FLATOW: Yeah. And that's how I - when I was a kid, or - that's how you look at an eclipse. You don't look at the sun, you know, you take a box and then put your head in it. You put a pinhole in it, right? A pinhole is like a lens.

LICHTMAN: Right.

FLATOW: And the sun will just focus on the paper. And we did that with -and you did that.

LICHTMAN: With a magnifying glass.

FLATOW: With a magnifying glass.

LICHTMAN: And you can do that, too, with a, you know, $2 magnifying glass.

FLATOW: Yeah.

LICHTMAN: It doesn't take much. This is a pretty easy experiment to do at home. And it's more than just a fire hazard (unintelligible)...

FLATOW: Yeah.

(Soundbite of laughter)

FLATOW: And no animals or insects are involved here.

LICHTMAN: No, we did - we strongly do not recommend involving...

FLATOW: Yes.

LICHTMAN: ...living things.

FLATOW: Yes. It's, you know, just the dry leaves are wonderful. Even the wet leaves you were using them on was 1,000 times the - wow. Thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: Yeah. That's Flora's Video Pick of the Week right there, up on our website at sciencefriday.com, where you can also find on our website you can find our podcast and, you know, everything else like that.

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