What kind of lens starts a fire

Press ESC to cancel. Skip to content Home Physics Can you start a fire with a concave lens? Ben Davis August 3, Can you start a fire with a concave lens? Which lens could start a fire most quickly?

Bottom: cross-section of Fresnel lens of equivalent power. Without getting too much into the scientific content behind how a Fresnel lens works, we'll tell you why you should care. A Fresnel lens can be paper-thin. You can carry one of these lenses in your wallet in place of a business card, and have a means of starting a fire as long as bright sunlight and dry tinder are also available. Emberlit gave us the business card fire lens seen above at the Outdoor Retailer show.

It's the size and thickness of a standard business card, and it's even flexible. If you want one, they're included in the Emberlit fire starting kit , or you can buy value-packs of these lenses from various online retailers. They weigh almost nothing, so you can easily store them in your wallet, bug-out bag, glove box, and various other key locations.

Pretty effective, but it's also possible to take this concept to the extreme. One YouTuber removed the Fresnel lens from the surface of an old flat-screen TV, and turned it into a monstrous degree solar torch.

Less portable, but a whole lot more powerful:. We're now offering a free digital copy of the OffGrid Outbreak issue when you subscribe to the OffGrid email newsletter. In the days of celluloid film this was a huge fire hazard in theatres and caused a shift to acetate base films as soon as the material was mature enough. Add a comment. Active Oldest Votes. This is explained in more detail in this earlier answer One thing to note - light bulbs have a minimum distance to the filament the bulb has a certain size both to keep the glass from getting too hot, but also to lower the risk of things catching fire because they are too close to the filament.

Improve this answer. Community Bot 1. Floris Floris k 12 12 gold badges silver badges bronze badges. The tricky part is that it really only works to the extent that the light source is a point source - and the closer you get to the light source, the worse it approximates to a point source fluorescent tubes are an obvious example. And while the source may not be a black body emitter, the thing you want to set on fire will approximate it.

Perhaps I oversimplified - but you will need a lot of Watts per square meter in your source to begin with , otherwise optics can do nothing. Show 4 more comments. Lawrence B. Crowell Lawrence B. Crowell Temperature at the target is created by light being absorbed. And in practice, people have started fires by accident with strong borderline illegal class 4 laser pointers that use sources that would instantly be destroyed if you brought them up to the temperature needed to start a fire.

The thermodynamic temperature of a laser emitting medium is in fact negative because it has been pumped such that there are more atoms in an exctited state than is possible to obtain by simple heating of the medium.

If you live in a hot part of the world, you may notice an appliance called air conditioner. It can make the inside of the room cooler than the outside, provided that the outside is heated more than the inside is cooled. It's basically to do with how much light you can focus in onto that spot of the sun, so you want essentially something which will work in the dark as much as possible - so as low an F-stop as possible.

A wide angle lens will work better and get more heat in that small space and cause ignition. But the amount of energy you can get in is limited by the radius of your lens so a big lens is better. The biggest effect I would say is you want to use a cheap lens because the focal length of these lenses - designed to focus onto a sheet of film at the back of your camera - that's probably only 2 or 3 centimetres away from the back of the lens. And so, any fire you start is going to be very, very close to your lovely camera lens, so use as cheap a lens as possible.

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