How to Set Something on Fire with a Magnifying Glass

SHI: Snails, ants and bugs? I say holocaust them.

Well, all boys kill insects. Or most of them do. Did you ever kill insects with a magnifying glass? It’s a blast.

You take a slow-moving bug and put it on the sidewalk on a hot sunny day. Then you point the magnifying glass at the bug and turn the glass a certain way so that its reflection becomes a single sharp point of light. I forget what is going on. You may have to converge two points of light together.

Anyway you get this single convergence of light, and it looks very bright and hot. You point this nexus of hot light at the bug, and shortly, the bug actually catches on fire! Yep, you are concentrating the sun’s rays so well that you can actually set things on fire. I assume you could set other things on fire this way such as dried grass or maybe even flammable wood. I think for sure you can burn paper this way because I think we used to set paper on fire like this.

What’s going on. Apparently the glass concentrates or channels the heat of the sun enough somehow that it the heat of the sun can actually start a fire. You can also use any piece of glass or even your eyeglasses! You have to keep moving that magnifying glass until you get that point of hot light down to a small dot about 1/4 inch in diameter. Only then will the sun’s heat be concentrated enough to start the fire.

The science itself. Involves things called photons which  carry the light (and heat) of the sun down to Earth. The magnifying glass is convex so it gathers the photons in a single place all together and the combined heat of the photons concentrated on that spot start a fire.

The temperature has to get up to 451 degrees (remember the Ray Bradbury book?) in order to start the fire. That’s hot enough to burn your skin so never put your finger in that spot of light and never point the magnifying glass at your skin and do this.

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One thought on “How to Set Something on Fire with a Magnifying Glass”

1. SHI says:

I learned this magnifying glass stuff in 5th grade science class but didn’t really kill anything. We were taught to use it to set a piece on paper on fire.

I’m really tempted to set something on fire now.

Quick quiz: if any of you could get away with a crime, what would you rather be? (NOTE: Just for fun. Not advocating any illegal action)

A) Arsonist
B) Slasher/knifing
C) Noose artist/hangman
D) Something more sicko

I’d choose A) every single time.