Making a traditional glass lens requires a lot of experience, skill, and patience grinding a piece of glass to the required shape, and is not for the casual experimenter. Making a glass Fresnel lens with its concentric rings requires even more work, but as the ever-resourceful [Robert Murray-Smith] shows us, a Fresnel lens can be made from far more mundane materials. He shows us a working lens made from transparent plastic tube, and even successfully smoulders a piece of paper with it under the anaemic British sun.
His lens, with its circular profile tube filled with water, is not perhaps the most efficient lens in terms of light focused per unit area of lens. From dredging up our highschool physics lessons we are guessing that half the light is diffracted outwards rather than inwards by the cylindrical profile of the coil, but for the cost of the whole device we’re not sure that matters. Next time we’re shipwrecked on a desolate island with a handy supply of clear plastic tube and fresh water, we know we can always raise a fire.
If Fresnel lenses interest you, we’ve taken a look in the past at their history.
Looks more like a diffuser with interference patterns.
I’ve also made a rudimentary Fresnel lens by printing a single layer circle with transparent filament on a 3D printer. Not a good one, but it worked. I tried to improve the design but optics is not a specialty of mine. 😂❤️
What about a cheap Nylon threat Like round shaped Trimmer Line. Roll IT Up between two plates. A little Bit solvend to disolve the surface and glued together. Let IT Degas a day. At the end you should have nylon-fresnel-plate.
You could use your 3d printer to make a mould and then fill it with clear silicone or resin.
I think one loop of the tube is enough becase all loops have the same convex shape and dont add to the concenration in the center.
That bit at the end look a bit suspicious to me..
I don’t think I’ve ever seen a glass fresnel lens (I’ve seen glass fresnel concentrators, like on theater lighting, but those are non-image forming). All of the sheet-type handheld ones I’ve seen were plastic. I got one from an old widescreen TV, was roughly a square meter in area, and it would focus sunlight down to a coin-size spot. It would instantly slump a stack of coins into a puddle and readily melted sand.
Lighthouses used big glass ones.
Lighthouse-sized Fresnel lenses are usually comprised of a lot of small chunks of glass, not one giant piece.
Rolling a tube like this will never bring you anywhere near a Fresnel lens.
And if it did, how would you get a focal length that’s that tiny fraction of the “lens” diameter? That bright spot was being focused by whatever was in the center.
Yeah, most of this lens is completely pointless
Having a liquid Fresnel lens could be interesting.
If it was for instance very large, being able to add something to the water to limit the transparency or change the refractive index could be advantageous.
If the tube were in the correct profile instead of round no doubt it would help considerably with the focusing.
Perhaps passing clear tubing through a heated die form might work to change the profile.
Profile needs to change as the radius increases. I don’t know what this thing is, but Fresnel would not want his name attached.
I tried to model cross section of this in 2D ray optics simulator: https://phydemo.app/ray-optics/simulator/
And it does not seem to work at all.
That’s no lens, let alone a Fresnel. Looking at about 5:30, what he’s got is the focusing effect of the surface of the inner coil, with the relatively bright spot being a caustic https://en.wikipedia.org/wiki/Caustic_(optics) due to the fact that this is very approximately circular.
It’s not a Fresnel lens. But it is a coiled cylindrical lens. And it is orders of magnitude less efficient at doing pretty much anything.
He should bond it to a piece of glass with optically clear degassed epoxy, leaving the top half of the coil above the resin. Face the flat side towards the light source and try mineral oil instead of water. That would be a closer approximation of a Fresnel lens.
For some extremely imprecise interpretation of the word “approximation”.
If you want a reasonable approximation of a Fresnel lens, then it should be possible to cut rings out of transparent plastic using relatively simple equipment, and to face and polish each one to the angle appropriate to its radius.
There’s really no excuse for a HaD writer to get this so grossly wrong, since https://hackaday.com/2018/05/11/antique-lighthouse-lens-via-cnc/ goes into it in detail and is by no means the only hit if one searches for “Fresnel”.
And the YouTube video is neither more nor less than clickbait for the ignorant.
Fresnel lenses, and whatever this is, work by refraction, not diffraction.
This is definitely fake. The shape of a Fresnel lens is basically the same as the shape of a normal lens with the only difference that plane-parallel blocks of the substrate (that do not really matter, since they do not contribute to the focal properties as e.g. a window does not) are removed to reduce the mass. The thing in the video however is like a rolled up cylinder lens. It will not produce a focal point but instead a spiral focal line relatively close to the tube. Therefore the light is not concentrated at a single point and you can’t burn anything with it. I guess for the burn-video they just put a small regular lens in the middle (it really does not have to be that big to burn paper) and that’s how they faked it. Also if you build a Fresnel lens that size the focal point would be much further away since you cannot have such a high Numerical Aperture with a single lens.