Burning lens, magnifier and reflector – fire from sunlight

A converging lens is thicker in the middle than at the edge. It bends the sun's rays so that they meet at a single point behind the lens – the focal point. All the light energy falling on the lens then lands on a tiny area. The bigger the lens, the more light it collects, and the smaller the spot, the hotter it gets.

Dark material absorbs light and warms up, while light-coloured material reflects it. That is why char cloth, charred material or very fine, dark, dry plant material catches best. A polished concave reflector – such as the reflector of an old torch – works on the same principle, except that it bounces the light back instead of letting it through.

Glasses can work too, but only reading glasses for long-sightedness, whose lenses are thicker in the middle. Glasses for short-sightedness spread the light out and make no focal point.

Hold the lens so that it faces the sun directly. Then slowly change the distance to the tinder until the light spot is as small and bright as possible. A large, blurred spot achieves almost nothing. Check the spot only briefly and from the corner of your eye.

Stillness is crucial. If the spot wanders back and forth, no single place can heat up properly. Brace your hands or elbows, for example on your knees or a rock. Shield the wind with your body, but do not cast a shadow on the lens.

The sun has to be strong. Around midday in summer it often works within seconds; with a low winter sun or thin cloud the method is much weaker or does not work at all. Keep your tinder bundle ready so you can pass the ember on straight away.

Eyes first: never look at the sun through a lens or in a mirror. The bright focal point itself is also very glaring – a brief glance from the corner of your eye is enough. Sunglasses make looking more comfortable, but they do not make staring at the sun safe.

The focal point burns skin instantly. Never aim it at hands, clothing or other people – not even as a joke. Children should only practise together with an adult.

Lenses, glass bottles and reflective surfaces can also start fires unintentionally when they lie in the sun. So always pack magnifiers and burning glasses away covered. Check the local rules and wildfire danger before every fire, because in sunny, dry weather – exactly when this method works well – the fire risk is often highest.

A flat Fresnel magnifier suits an emergency kit. It consists of a thin plastic sheet with fine rings that act like a thick lens, weighs almost nothing and fits in a wallet. A classic glass magnifier is sturdier but heavier. Both should be packed so they cannot get scratched, because scratches spoil the focal point.

A round container or bag filled with clear water can act like a weak lens. That is a makeshift solution and much less reliable than a real lens. As tinder, char cloth works best because it smoulders with very little heat.

Burning glasses have been known since antiquity. Military survival manuals list the lens as an ignition method but stress that it only works in bright sunshine. Its big advantage: it uses nothing up. As long as the sun shines, you can make fire again and again.

A burning glass usually produces an ember rather than a flame. So you always need a good tinder bundle and thin kindling to turn it into a real fire. And because the weather has to cooperate, a lighter or ferro rod always belongs in your pack as well.