How to Set a Fire With a Magnifying Glass & Paper

By concentrating sunlight on a small area, magnifying glasses can start fires.

Magnifying glasses, which are convex lenses made of glass or plastic, work by creating a magnified image of an object, so long as the distance between the object and the lens is less than the focal length of the lens. Past this distance, the lens shows a smaller, distorted image of the object. Using these principles, a magnifying glass can be used to focus the sun's rays onto a small point. The bigger the lens, the more sunlight it can focus, and the higher the magnification, the smaller the point it can focus the light into. With a little bit of practice and patience, a magnifying glass can easily burn paper or char wood.

Things You'll Need

• Magnifying Glass
• Paper

Instructions

1. Hold the magnifying glass so that the surface of the glass is perpendicular to the sun's rays. In other words, from the point of view of the paper you're trying to burn, the sun would appear straight through the magnifying glass. Do not actually look through the magnifying glass at the sun, as you could be blinded forever.

2. Look at the circle of bright light which forms under the magnifying glass. Tilt the glass until it becomes a perfect circle rather than some sort of ellipsoid, like the one in this article's picture.

3. Move the glass closer or further from the paper until you find the distance where the little circle of light is as small and bright as possible. Keep the angle of the glass in the position where the circle remains a perfect circle. This gives the most efficient concentration of light. Do not stare directly at the bright circle for long periods of time. Very powerful magnifying glasses can hurt your eyes, even with reflected light.

4. Hold the glass so that the tiny circle of light remains steady on the surface you wish to burn. Once the paper starts to burn at that point, move the lens around a bit to heat up a larger area of paper. With any luck, it will burst into flame.

Tips & Warnings

• Never look through a magnifying glass at the sun. Looking at the sun with the naked eye can be painful but usually isn't permanently harmful. Looking at the sun with binoculars or any sort of magnifying lens is likely to cause permanent blindness.

• Never leave a fire unattended. Put out any smoldering objects with water. Keep water nearby whenever you make a fire.

• Never leave pieces of glass in the woods. The same properties that allow a magnifying glass to start a fire can also lead to one caused by an unattended piece of glass discarded in the forest.

References

• Wildwood Survival: Fire - Magnifying Lens