Instructables

Giant Fresnel Lens Deathray: An Experiment in Optics

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Step 11: OK - Let's Burn Something!

Update: at the suggestion of foobaz utne, I flipped the Fresnel lens around so the ridges are facing the sun, and found a dramatic increase in lens performance. The secondary lens still isn't working right, but I was able to upgrade from melting zinc pennies to liquefying solid copper ones and destroying nickels!

First Test

Inspired by similar Fresnel experiments floating around the net, I decided to try melting a penny. On winter solstice, I found that a zinc penny melts within a minute when held in the focus. Solid copper pennies (from 1982 or earlier) wouldn't melt, but probably would during summer. Copper's melting point is almost 2000oF compared to Zinc's 790oF. See the first row of images for these tests.



Round 2

With the Fresnel lens oriented correctly, I had another crack at melting those coins. The following video and the second row of images shows my results. MUHAHAHAHA!!!



Note: Copper's melting point is about 2000oF, but Nickel's is 2600o. So it's highly possible that only the copper in the coin (75% copper, 25% nickel) melted, resulting in the mutilated pitted surface.
 
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sleepydog1 year ago
Great job! The build detail, thought process and references are outstanding. I'm working on a related project and this helps me immensely. Thank you for taking the time and effort to share your work.
Mike McGill4 years ago
Hi, This is very impressive. I reckon it would make an extremely clean method of melting metal for casting, using a cast iron crucible at the focal point. I remember reading years ago about an experiment in Switzerland, where a hillside was covered with parabolic mirrors controlled and focussed by computer. They burnt a hole through a six inch steel plate in less than a minute.
d-sier6 years ago
When you heat copper you make it soft, very useful when you want to shape it (without cracking it). Its used on most metals, the tell-tail of copper is that it changes color when you do it. (If I remember correctly the color change is because you burn away the outer corrosions) if you want to make it hard again you heat it again and then cool it quickly (drop it in cold water). Try hitting it and a un-heated coin with a hammer and you will see the difference ;)
aleksei d-sier5 years ago
you're talking about work-hardening, right? I just learned that in metal working.
NewDLee d-sier5 years ago
Also the layer under the charr of a copper sheet is photovoltaic! Home-made solar panels anyone?
josheeg5 years ago
Hi I know people would be very interested and I would to in this idea if I could know what to expect based on the lumens or what I see on a light gage. How would someone calculate that?
downhere6 years ago
would it work if you put flexible optical fibre at the focal point to enable you to point this super evil device at a moving target, just to make it more sporting?
DrSimons (author)  downhere6 years ago
I thought about that, and indeed it'd be cool....however I have a strong feeling that no matter how good the optic cable was, it would get melted....
kd1s6 years ago
This gives me an idea. Why not put a coil of copper wire in the path of the focal point and then use steppers to control angle of the fresnel along with rotation of the entire platform. Then circulate cold water through the copper and free heating! Of course at those high temperatures you'd need to have a fairly fast flow. But hell, it'd beat paying $3 or more a gallon for oil.
iq_abyss kd1s6 years ago
I have a feeling that a variation of this will be implemented on my new house.
I wonder... if you took a glass rod with both ends cut flat and perpendicular to its direction, and mounted it with one end right at the focal point, would it straighten out the beam? I bet then you could make a movable gun using a complex arrangement of gears and mirrors. bwahahaha