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http://en.wikipedia.org/wiki/Fresnel_lens
A Fresnel lens is a type of lens invented by French physicist Augustin-Jean Fresnel (pronounced fre-NELL in scientific and lighting applications. Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances.
When placed in the sun, a fresnel lens will act as a giant magnifying glass and concentrate light to a very small point. Most large fresnel lenses will concentrate several square feet of sunlight to less than an inch resulting in a hot spot over 2000 degrees Fahrenheit. This will cause wood to instantly catch on fire or zinc and copper metal to melt in a few seconds or even burn and vaporize. We have boiled 12 oz. of water in a dark glass bottle in 90 seconds and burned a hole in a stainless steel bowl.. One gallon of water was boiled in 30 minutes.
HELPFUL HINTS:
The sun is always moving across the sky (earth rotation) and the angle of the lens must also change for maximum results. It is helpful to have a table/stand that can be adjusted up and down.
WARNING:
Never place hand in beam, instant serious burn. Surfaces and objects can get to 2000 degrees. This will send you to the ER. Never experiment without protective gear. Gloves (fireproof). Never leave lens in the sun unattended or you could start a fire. Never let kids play with the lens or use while pets are around. WEAR PROTECTIVE wielding eye wear. The spot formed by the lens is equivalent to looking at the sun. Vision loss is possible without proper eye wear. Some objects have remained hot for over an hour even out of the light. It can be quite deceiving how strong the sun is and anyone using these lenses for sun collection should get in the habit of treating in like a 2000 degree furnace or blowtorch.
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