Step 4: Measure the Focal Length

Rather than proceed with building the rest of the device, at this point you need to measure your lens' focal length. This is the distance from the lens to the spot of light it will produce. This distance will only be correct when the light rays hitting the lens are parallel to each other, and perpendicular to the lens. In other words, the light has to either be sunlight or two parallel laser beams, and hit the lens dead on. Unless you live at the equator, with the sun straight up, measuring the focal length is actually quite difficult. After a lot of frustration I decided to use lasers.

Materials Required:

  • 2 (or more) laser pointers
  • A level
  • Some flat ground
  • A T-square
  • A tape measure
  • A large, rigid screen

We want to find the point in space where parallel light beams bending through the lens intersect. This is the focal point, and it will be straight out from the center of the lens.

Laser Setup

To set up our parallel beams of light, put your two laser pointers on either side of a book or something so that they're parallel. The goal is for the lasers to be perpendicular to the lens, so make sure they're on a level surface. Turn them on and aim the whole setup straight at the lens.

Meanwhile, have someone hold the lens straight up, using a T-square to make the lens perfectly vertical. You'll get two weird diffraction patterns on the wall behind the lens.

Finding the Focus

Now, with your tape measure extending out from the base of the lens, hold your screen up so the two lasers hit it. Move it back and forth until the two spots converge. When they do, see how far from the lens your screen is.

This may sound confusing, but the pictures should help. I recommend trying several times, maybe moving the lasers around, so you can see whether your results are consistent. For my lens, the focal length was about 40 inches (about 100cm) which is average for especially large Fresnels.
I bought a lens on a stand at a rummage sale, they said it was used to magnify a small screen. That sounded fun. It was big so I left it on the porch. Next day I noticed someone must have tried to set our porch cushions on fire! A day or two later there were more burns in the porch cushions. Eventually I figured it out....when i think of what could have happened because i didnt realize what was happening. <br>That said, it is very cool, but not for the uninformed.
<p>@SelenaH5 </p><p>Please read this page again and again and again until you understand it.</p><p>1. The lens focuses a large area of sunlight to a fine point. Whatever is at that point gets very hot, even just air, it starts to glow brightly and start to burn - even metals and glass will melt, concrete burns. This includes you, your hand, your brain if you're that uncautious. This is dangerous stuff.</p><p>2. The glow can get as bright as the sun, after all it comes from the sun, and that will damage the eyes of anyone looking at it. You do not want to blind anyone, a school is fir the education of the ignorant, and it's likely someone who's ignorant will ignore or not understand any warnings you put up, Never underestimate the capacity of the human being to be stupid, Health and Safety is designed to plan for these as among the vulnerable.</p><p>3. The project may cause other people to try to emulate it without knowing about the need for safety precautions.</p><p>So, just as a school is unlikely to allow someone to work with thermite, which is well within the capacity of most children to put together, equally it should not allow anyone to do this with a lens larger than 30cm across. Working on this scale is withi one order of magnitude in danger, and that's far too dangerous to be allowed in a school project, sorry.</p>
<p>Hello, thank you so much for this experiment! My sister really wants to step up her science fair game and we are thinking of doing something like this. Do you think that we could melt a penny (or anything) with just the fresnel lens? </p>
<p>Dont throw away they TV itself. The rear projection tv's have three tubes full of a bunch of lenses, long focal point, short, convex and the like. very high quality optics too, good for refractive telescopes.</p>
It may not work on a solar panel but it it should enhance the efficiency of the passive solar heater I have been thinking of building to help heat my house. By building a metal frame with soda pop cans stacked inside and a clear front and mounting a lens like this in front of the glass front I should multiply the amount of heat collected from the air flowing past the stack of solar heated aluminum cans. I just realized it would only be in focus a small part of the day since the solar heater box will be mounted on the south wall of my house. Still it shouldn't hurt anything. I might have to make the back of the box out of heaver metal.
Be careful with that, Aluminum melts at 1200 F... (Copper at 2000, and it _destroyed_ the penny) It could well work if it's not in perfect focus, just be careful and don't burn a hole in your house. Though, a well designed solar furnace to melt metal would be awesome. If I can find one of those lenses anywhere, I'll see what I can do, and if it works, post an instructable.
that was a similar idea that i had. use fresnel lens to heat metal ffor forging and smelting. no need for coal or wood. just stick the metal bar under the light for a few seconds and then continue forging. i've heard of parabolic mirrors being turned out of alluminum (lathed.) why? it would be simpler to just take molten metal and pour it on on a spinnin disk. the liquid metal would take a parabolic shape, and you would not need an insanely large lathe to make it. i'm seeing 12 foot mirrors made with an old car motor.
<p>There were telescope mirrors made by spinning a mirror with a coating of liquid mercury on it, in the days before they realized mercury vapours were poisonous.</p>
please elaborate on your pouring molten metal on a spinning disc idea, whichever way i picture this, i always end up with molten metal being thrown out from the edges of your parabolic shape towards faces, via inertia and gravity.
the disk would have to have edges.... like a pie pan shape. my comment does sound kind of silly, doesnt it?
<p>I would agree. Molten aluminum poured on a spinning round disc more like a cake tin (perpendicular edges) than a pie tin (angled edges). It would push most the molten metal to the edges and give a curved shape. The speed would still need to be controlled enough to spin the metal to the edges enough to make the curved shape but slow enough not to spin it all to the sides or out of the mold. But a 12 foot parabolic mirror would be easier made from an old satellite dish than spinning molted aluminum. </p>
lol its algoods, it just sounded like an alarming methodology, with the oversimplification on a site full of people ready to follow instructions down to the letter. and yeah i think they are used in certain telescopes or something, mercury mirrors, i think that when you're getting close to 12 feet its trickier to control the depth of the curve precisely via the speed of rotation because of all the stuff to do with the fluid dynamics and inertia and friction and suchlike. would be fully awesome to use a fluid mirror to reflect energy to heat a boiler that powers a steam turbine that drives a generator that charges a battery which drives the motor for the mirror as well as a surplus charge.
..... if only that was possible. ... also, for all you 'down to the letter' people, please look at my user name. my ideas usually are dangerous even with COMPLETE instructions.
well i must admit, the fun is usually directly proportional to the danger.
Thats some intense heat.<br>You could purpose that heat for electricity in 1 of 2 ways.<br>Stirling engine, Steam turbine !!
Both of these ideas have been done. There's a Youtube user called GREENPOWERSCIENCE that has posted videos of these in action. He plays with all kinds of solar. In his most recent video, he cuts a beer bottle in half by lightly scoring it, then using a small parabolic mirror to stress the glass. It's neat.
i found a source of the lenses. craigslist. search for rear projection tvs. most are free for the haulin, and over 55 inches. start the burnin!
Yes . I got 2 from curbside garbage also. Right now with black friday ,these things are all over the place like old CRT's . Maybe there should be a mandate to collect these lenses for 'green' industrial use ,or ship to 3rd world countries for use. <br><br> I wonder if the lens from old seeing glasses could also be used for the focal lens in a pinch .
I took apart an old projection set about a year ago and just stashed the parts for eventual toy's. The one I had used three CRT's, Three hefty projection lenses and a front surface mirror ( which a friend broke! ). Lot of other stuff like a blower and hoses used to keep the center tube cool, the screen and lots of other parts of various types. <br> <br>No fresnel in this type. I am looking for things I can do with those monster lenses. <br> <br>I am sure I could put my smart phone in the right spot and have a large screen phone but since I cannot use touch screen that way, the only point would be to make a holder for the phone and perhaps use it to watch streaming movies on the wall in a dark room. ( But then I have HD TV for that too ! ) <br> <br>Any other good ideas to use them?
<p>I find myself in the same situation! I was lucky in that the RCA I found sitting on the curb awaiting the guys in orange plastic suits had a Fresnel lense. I, as usual, salvaged the whole thing. I held up one of the three projection lens to my wicked lazer (the brand name, believe it or not) and it made the beam much larger, the point now being a perfect circle of light. But, like you, I am sure something really cool can be made from these things. Is there any way to safely get the concave one out of the tube??</p>
Here is image of one of the lenses. All three same. They include the aluminum mounting flange that mated to the front of the CRT. Weigh 8LB each. About 5&quot; Dia. Overall assembly is about 8.5&quot;. Lens screws in/out to focus.
<p>Where I can see this coming in handy is in the use of a water/glass heat-sink/storage in a greenhouse to maintain temps during sub-freezing nights, and with very little effort.</p>
<p>Loving the instructions. Forgive me if I sound like an idiot... but I know nothing about lenses or x-ray lenses inside these TV's. Should concern be given to the warning label on the smaller lenses in the aluminum casing inside the tv I tore apart?</p>
<p>x-rays are only generated when the device is powered, because it uses an electron gun powered by high voltage. make sure to discharge everything before touching any metal part or you could die! crt's store lethal charges inside during a long time</p>
<p>What I find exiting is the idea of melting silver or gold for casting jewelry without having to spend money on gas and or oxygen tanks. Another great idea would be firing precious metal clay(PMC) with this! Look it up. I'm sure you'll find something fun to make. They even make a copper version which is a lot cheaper.</p>
<p>Has anyone tried an expander lenses coupled with a corrector lense? This is typical with &quot;lasers&quot;</p><p>https://www.google.com.au/search?q=laser+expander+corrector&amp;safe=off&amp;client=ubuntu&amp;hs=5pa&amp;channel=fs&amp;source=lnms&amp;tbm=isch&amp;sa=X&amp;ei=nc3XVJXwJ8XZ8gWLxYCAAQ&amp;ved=0CAgQ_AUoAQ&amp;biw=1709&amp;bih=965</p>
<p>Just wondering, wouldn't the focusing lens melt when placed a the focal point of the Fresnel. Thanks.</p>
<p>no, because it is transparent and does not absorb light. The only way light energy becomes heat energy is when it interacts with an object that <em>absorbs </em>light, converting the light to heat. that's what happens when you put an opaque object in the path of this solar death ray. Now, if you have a poor quality, plastic lens with impurities, that will probably absorb some light and might get hot, possibly melting if it's a <em>really </em>crappy lens. Stick with glass lenses and you'll be fine.</p>
<p>Wow! I have to say that this is one of the best examples of what I hope for from and instructable! You've done a great job with the explanation of the scope of the project, the theory, learning from mistakes, and a good summary. Really well done step-by-step directions too. I work in a science museum and often need to explain the way that optics affect light - your examples are so well done! Thanks also for the reference link to the Java apple about optics. Keep up the great work, and thanks for sharing this! </p>
<p>NICE! I love this Instructable!!</p>
<p>I followed this instructable pretty closely, and can confirm that it works exactly as advertised. My lens does not focus as finely as DrSimons, nor have been able to mount a second focusing lens, but it easily gets hot enough to ignite lumber, fry eggs, or boil water in just a few minutes.</p>
i have been using welding glasses and now im ordering some of these: http://www.amazon.com/gp/cobrandcard/marketing.html/ref=cobrand_ch_t1_huc_40_mr?pr=conplcc&amp;inc=plccgatefomgc&amp;ts=cbuvwuwiaylwkym2wb4yadu6yjs965y&amp;flavor=EXCFIN&amp;plattr=math&amp;ad=PL10&amp;place=huc&amp;imp=A183TY2Y75EU1Uhttp://www.amazon.com/gp/cobrandcard/marketing.html/ref=cobrand_ch_t1_huc_40_mr?pr=conplcc&amp;inc=plccgatefomgc&amp;ts=cbuvwuwiaylwkym2wb4yadu6yjs965y&amp;flavor=EXCFIN&amp;plattr=math&amp;ad=PL10&amp;place=huc&amp;imp=A183TY2Y75EU1U <br> <br>im gonna tape a pair to a string on my fresnel and pass the rest out when doing a demo. also it might work if you use a mylar space emergency blanket taped into a circular construct around the focal point to shield the radiation because of the aluminum just put tape on the back of a space blanket and make it into a circle and beam the light into the center of that
what an interesting webpage can i view this from portal.unn.edu.ng
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.
OK, the big electricity companies will poo poo it, but I'm thinking that this could be a cheap approach to desalination by condensation. The steam could simultaneously be used to supply the water vapour, and also spin a generator to pump cold sea water through the condenser in one swell foop. Stopping at night is irrelevant as the output is storeable. <br> <br>Love this site, and its participants. <br> <br> <br>
Were you ever able to work out the problem with the secondary lens ?&nbsp; It ,may be that some form of lasing element would work.&nbsp; The guts of small laser pointers which are almost free may be worth trying.
Trying to be nice here, but what part of physics and engineering are you actually basing this comment upon?
This would be awesome with bugs crawling everywhere. <br>KILL IT WITH FIRE!
Just a, maybe wrong, idea: what about adding a reverse Fresnel lens (as small as the focusing) at the focusing point? Won't it reverse the light in a straight path (concentrating the light)?
Did you see the article where the Israelites concentrated the beam into a glass fiber optic cable and was able to use in in surgery?<br><br>http://www.newscientist.com/article/dn4009-optic-fibre-delivers-solar-surgery.html<br>http://www.newscientist.com/articleimages/dn4009/0-optic-fibre-delivers-solar-surgery.html
Here's how I&nbsp;found the focal point...&nbsp; Get a trigger pump type of fluid sprayer, either a glass cleaner sprayer or you can buy empty plant misting devices at a hardware store. Put the lens in the sun and spray water behind the lens. You'll see the rays converge and diverge. I guess you could measure it with a metal tape.&nbsp; <br /> <br />
I really like the misting idea, would be great in demonstrations with my Fresnel lens cooker.
I use a regular polarized sunglasses and wear the Real circular polarized 3D glass in front of the other pair, they darken up quite nicely and I always have sunglasses on when you lift the Real to undarken you view. This is the only way for the two polarized glasses will work, put them in any other order or angle and it will not get any darker.<br><br>Polarized lens are so cool to work with.
What type of glasses are recommended for a fresnel lens? Would these be adequate? Please answer only if you actually know, I would prefer to keep the remainder of my eyesight. <br><br>http://cgi.ebay.com/Pro-Rider-IR3-Welding-Wraparound-Safety-Glasses-Z87-1-/330526133677?pt=LH_DefaultDomain_0&amp;hash=item4cf4e48dad#ht_6224wt_1139
Firstly, I don't have much formal welding training and I can't tell you the specific ratings to look for. What I can tell you is that I used goggles designed for gas welding (ie oxyacetylene), and I still saw a lot of vision spots after staring at the focal point for a little while. It depends a lot on how much exposure you get. If you really want to be able to stare continuously at it, go with something rated for arc welding. Usually you can't see anything through these other than the actual bright spots. The goggles in your link definitely aren't dark enough for that, they're definitely better than nothing.
--NOTE!!:<br/>1) Not all big boxy televisions are rear-projection! Smaller and especially older ones may simply be CRT (not rear-projection) which means there is no screen, just a metal cathode inside a solid glass tube. On the inside of the front face of the tube is a very thin and very toxic shiny phosphor coating. There is no lens, other than--perhaps--the glass itself. I repeat: what you are looking at--the outermost layer of the display--in a CRT television is the solid glass tube itself.<br/>2) UNDER NO CIRCUMSTANCES BREAK ANY GLASS IN THE TUBE<br/> If you have to break glass or ceramic sealant, you're doing it wrong.<br/> The toxic phosphors will flake off and start floating around like dust, it *will* get everywhere.<br/> Yttrium (in the red pixels), for one, causes lung disease.<br/><br/>Some tubes have a band of metal glued to the tube which looks like it is holding a screen to the tube. They are not. While it is possible to remove it, you will find nothing more behind it than a solid glass seam which is part of the main tube, not a seam holding the tube to a screen.<br/>
Who cares? if you get a crt by mistake, you've just gotten yourself a flyback!
i totally agree, and a big one at that
Could you show photo how looks CRT television the solid glass tube,please?<br><br>If you can, then please send it to my blog:elektritsaabtasuta.blogspot.com or mail me sullivanbrendan35@yahoo.ie
Basically, if you can't tell the difference between a CRT and a rear projection tv then this and almost every other project on this site is not for you.

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