| Author | Message | ||
     gregory |
With a regular fresnel lens, about the size of one from an overhead projector, would it be possible to melt pennies? Or does it have to be a huge lens? | ||
| Simon Quellen Field (Sfield) |
You get about 1,000 watts per square meter of sunlight at the ground on a good day. A lens of the size you propose would be about 1/9th of that, or 1/10th. With at most 111 watts of heat, you are unlikely to be able to melt a penny. | ||
| Anonymous |
can you link fesnel lenses together to get more energy? | ||
| Simon Quellen Field (Sfield) |
Yes and no. Two lenses side by side will gather twice as much energy as one. However, they will focus the energy onto two separate spots. You could get clever with a couple mirrors and try to focus each spot onto one place, or use one mirror to aim the sun at the second lens in such a way that both lenses focus onto the same spot, but it is much cheaper and easier to just use a larger lens. | ||
| AnotherAnonymous Unregistered guest |
Sometimes you can find one in an old rear projection TV. These are often found when people decide to upgrade to HDTV flatpanels. Some focus the light to a line, but sometimes you get lucky and find one that makes a spot. I acquired a scratched 62" one from a dumpster (3x4 feet approx), and have yet to find anything that doesn't get destroyed in its beam. Melts: steel, ceramic, most rocks, every coin I've tried Burns: asbestos, tungsten, aluminum, flesh Explodes and then Melts: quartz, obsidian, mirror (slowly) Explodes and then Burns: soda can (fun) Boils: lead, silver Graphite burns very slowly, so makes a useable crucible material. An industrial diamond is the only thing I was unable to destroy visibly. I am still trying to acquire some rhenium for such a test. As for combining beams, after some experimentation and some geometry, I concluded that the only practical way to do it would be to redirect the light prior to focusing it through the lenses. IE, set up (for example) 4 large lenses each with their focal point on the target, and then direct 4 beams of sunlight (with mirrors larger than the lenses) at the lenses. However, it just seems easier to find a bigger lens. | ||
| AnotherAnonymous Unregistered guest |
Oops, forgot to mention. The biggest problem with bigger lenses is the cost of shipping. One, about the size I have, is about $100 from a southern cal website, but the shipping is $50. Even worse was that a friend in Sweden wanted one, ouch! I considered driving down and buying one, bringing it back, cutting it up and labeling all the parts, so it could be reconstructed (and shipped very cheaply), but then I discovered: http://www.goldmine-elec-products.com/prodinfo.asp?number=G13994&variation=&aitem=2&mitem=3 Only drawback is, it is *so* thin, you either have to build a rigid frame for it, or squish it between two sheets of something (plexiglass works good), or possibly just tape it to one sheet. | ||
| lysdexia Unregistered guest |
Fresnel lenses crack too easily. *sigh* It melted rocks?? how? Wouldn't you need to coat the target in black to stop all the sunlight from bouncing off? Anyone know how I could just make a good Fresnel lens? | ||
| AnotherAnonymous Unregistered guest |
What have you been doing to them to crack them? And, it easily melted/cracked almost any rock, even white quartz. About half the light hitting it is infrared, plus the black graphite I use for a base absorbs most visible light and transfers it to the rock as well. I can even break/melt a mirror after a while. If you had a really good CNC mill you could possibly program it to etch a piece of plexiglass. The wider the ridges are, the more heat you get to the target (smaller ridges = better optics = more light lost), so ridges the size of your target would be optimum. You'd need a large and thick piece of plastic to save money doing it this way. However, if you want to get that big, it is probably better to set up a mirror array, or coat the inside of a 12 foot satellite dish with aluminum foil. Obviously if you have enough time to correct all the spelling mistakes on a board frequented by kids doing science projects, you have time to do that. | ||
| lysdexia Unregistered guest |
The cracked one was a page magnifier I'd stepped on under and over a pile of papers and carpet when I was looking through a mess. The crack by now went most of the way, so it isn't in two pieces, yet. I bought a round lens over twice as big from a reverse auction site. It has a squarish frame, so it must be a projector stock. That one's fine. To my great disappointment, "Al" foil or cans do not burst into searing flame if they're brought into a butane flame, even if I snipped them into tiny threads. Are they alloyed with some stupid elements to prevent me from using them as a fuel? They hardly burn, and mostly boil. | ||
| AnotherAnonymous Unregistered guest |
Well, stepping on them will do that I suppose. Take a piece of 1/8" plexiglass cut out to the same size, and glue or tape it to the lens, if you want a stronger setup. The aluminum doesn't "burst into searing flame" in my lens, but it does oxidize rapidly, depending on the amount. If it is a big piece it will conduct most of the heat away from the focal point, and just get a bit soggy. Why do you want to use aluminum as a fuel? Try magnesium if you are just after some pyrotechnics. Aluminum can after exploding in the lens:  And some more random pics: 3/32" steel after a few minutes under the lens:  Graphite crucible after a minute or two (taken into a dark room)  Fire brick, soapstone, and ceramic crucible (in the back is a graphite block)  (when I first tried to melt metals, like nails and coins on random bricks and rocks, and even a chunk of asbestos, most of the time all I would do is weld the metal to the glassified rock) The lens itself:  | ||
| Simon Quellen Field (sfield) Senior Member Username: sfield Post Number: 283 Registered: 12-2004 |
Liz: Coat a fresnel lens with mold release, then cast a positive by pouring acrylic resin onto the original. Now you have the mold, and can cast as many as you like from it. | ||
| AnotherAnonymous Unregistered guest |
It seems like the surface tension of anything you can cheaply cast with (that would save money over just buying a lens) would limit ability to make an accurate duplicate. Most of the larger (more than 12") lenses I have used, have grooves around .005" apart. Unless you have access to a glass fresnel array from a lighthouse. However... The smaller the grooves, the better optical quality you have. But the larger the grooves, the more light gets through. All depends on what your definiton of "good" is. If you could find a large crude (wide groove) lens, it might be cheaply copyable with casting. Does anyone out there makes fresnel sheets with much wider grooves (around .25 inch) specifically for solar collection (not mirror arrays or parabolic mirrors) rather than optics? I am not sure. You might have to make one. If you do decide to make one, the grooves could be (at most) the width of the target you wish it to focus to, I suppose. Aka: if you want a 1cm diameter target, then 1cm wide grooves would be your limit. But if your focal point is close, the lens itself will start to get a bit thick. | ||
| Simon Quellen Field (sfield) Senior Member Username: sfield Post Number: 297 Registered: 12-2004 |
The lenses used as magnifiers and projection lenses are pressed like phonograph records. But casting resin can easily reproduce much finer features than the lens has. I have used it to make casts of cells on plant leaves. Just brush it into the grooves with a paint brush. Works great. | ||
| AnotherAnonymous Unregistered guest |
That method may work, if you can acquire a resin that is optically clear (enough), won't shrink, and won't make an overly weak lens (water clear urethane?)... and if you are patient enough to paint the whole thing without letting any bubbles get trapped under.... and if you can find a molding agent (to make the negative casting) that has the same quality, but also will not stick to the resin or original (silicone?). But can you do it cheaper than you can just buy more lenses? You have to buy or borrow a lens to copy first of all, so you'd probably have to make and use/sell several copies to save money that way. 11x11" for $5: http://scientificsonline.com/product.asp?pn=3070877&bhcd2=1113693248 31" x 40" for $45 (rolls up, low shipping cost): http://www.goldmine-elec-products.com/prodinfo.asp?number=G13994&variation=&aitem=2&mitem=3 38" x 50" for $100 ($56 shipping, but if you buy more, shipping is less each) http://www.alltronics.com/lenses.htm | ||
| catbed Unregistered guest |
does eckerd sell fresnel lenses? | ||
| krs (Atomico) Junior Member Username: Atomico Post Number: 4 Registered: 8-2006 |
can you melt anything from steel with the fresnel lens and put it into a airsealed potion and then you have liquid steel and you don't have to buy for $40 | ||
| Simon Quellen Field (Sfield) Senior Member Username: Sfield Post Number: 1433 Registered: 12-2004 |
So, you want to heat some steel until it is white hot and molten, then pour it into some magic container that will insulate it well enough to keep it molten, and still survive the 1400 degree Celsius temperature? I'll bet that container costs more than $40. |
