DANGERR Burn Things with a Magnifying Glass
TIME: 30–60 MINUTES COMPLEXITY: EASY
YOU WILL NEED: Magnifying glass, available at drugstores » Scratch paper » Fresh fruit
WARNINGS You are responsible for extinguishing anything that you ignite. Always work in a clear area where you can’t accidentally ignite anything else. » Light hot enough to burn paper can also burn you — so don’t focus it on anything you don’t intend to ignite.
NOTE Treat the magnifying glass gently. The lens can be scratched easily, reducing its effectiveness.
SUPPLEMENTARY DATA
The speed of light is a constant — in a vacuum. Light travels at different speeds through different materials, so when it goes from one to another (from air to glass) it changes direction in a predictable way. This phenomenon is called “refraction” and it’s what enables a lens to focus light, or drops of water to make a rainbow.
It seems remarkable that anything is transparent. After all, glass is more dense than wood and yet somehow visible light can go right through it. Air, water, plastic, and certain minerals are about the only substances transparent to visible light. However, everything is transparent to some form of electromagnetic radiation. Our bodies are transparent to X-rays, the planet is transparent to cosmic rays, and paper is transparent to microwaves.
Harness the awesome power of the sun.
1. Prepare. Find or make an area that’s free of flammable materials. A sidewalk, driveway, or dirt path is ideal.
2. Focus the light. Hold the lens above the paper and notice the bright circle of light that it makes. Move the lens up and down until you make the smallest circle of light possible — this is concentrated sunlight and it’s very hot.
3. Burn. Hold the lens still and observe the effect on paper. Try the same procedure on fresh fruit. Write your name on a wood scrap or melt a bit of plastic. Experiment.
The amount of heat you generate at the focal point of the lens depends on the size of the lens and the angle of the sun in the sky. The atmosphere absorbs and reflects some of the light. When the sun is low on the horizon, the light must travel through more of the atmosphere to reach you. 
Excerpted from Fifty Dangerous Things (You Should Let Your Children Do) by Gever Tulley with Julie Spiegler (fiftydangerousthings.com). Gever is co-founder of Brightworks, a new K–12 school in San Francisco (sfbrightworks.org).