It’s strange to think that the light of the sun can light fires here on Earth, but with the right concentration of sunlight — it’s surprisingly easy.
For those of us who burned ants with magnifying glasses when we were mischievous children, we already know one way to make a white-hot spot of burning heat with a lens, but did you know there are other ways to turn sunlight into fire?
1. Burn with a lens.
There are some real benefits to a magnifying glass. It can last indefinitely and light an indefinite number of fires — when the weather is sunny and clear. Just focus a blinding pinpoint of light on the flat spot in some dark tinder, angle the lens to make the dot of light as small and round as possible. Manipulate the lens at different angles and at different distances from the tinder until you have the perfect "dot" and the smoke should start to flow from this hot spot immediately. Blow gently across the tinder while you are magnifying light on it. Your extra oxygen and air movement will cause burning fibers to spread their ignited red glow into neighboring fibers. This is the birth and expansion of your tiny coal. This growth will allow you to go from a speck of light and a little bit of tinder to a tinder-born fire.
2. Burn with a parabolic mirror.
With a curved mirror, like the mirrored cup from a large flashlight, you can also create a focal point of heat and light. This is a little bit different from light passing through clear lens to be concentrated. With a parabolic mirror from a flashlight, concentrated sunlight forms a blinding white-hot point inside the cup. This means that you’ll need to place the tinder where the flashlight bulb once sat when using a reflecting cup. More shallow concave mirrors — like the bottom of a polished aluminum soda can — will have a focal point a few inches away from the bottom of the curvature. Whichever shape you use, this technique will work the same way. Move the mirror into position so that light focuses on some suitable tinder, and the flammable stuff will start to smolder. The red ember you create can then be blown into flame when combined with more tinder.
3. Burn with flat mirrors.
This is a remarkably old technology. The ancient Chinese were lighting fires with mirrors before the birth of Christ, and the famous inventor Archimedes was said to have developed and used a solar burning weapon against incoming ships at the siege of Syracuse in 212 BC. To make this work in modern times, you’ll need many small mirrors (about a dozen, depending on the size) and some excellent dark tinder (like char cloth). Start positioning mirrors to bounce light onto the tinder and keep adding mirrors in a curved arc around the tinder until it starts to burn. Watch your fingers! This method can easily burn your skin, so it’s best if your square of char cloth is impaled on a piece of wire stuck in the ground — and keep your hands and body behind the mirrors when using them. Dismantle the set-up as soon as you have your fire.