fresnel lenses

I have a cheap fire starter, a big magnifying glass, and a small loupe There was a survival program on TV where a guy used a piece of ice in the right form as lens to make a fire.

Rubbing 2 pieces of wood works too.

I have some sheet from ebay too, 2015, experimented with it in 2015 Full Page Magnifying Sheet Fresnel Lens 3X, like this (different seller I used):

Doesn't water absorb infra-red?

They can work up to a point and are very handy by being flat. Keep it in a soft envelope to protect from scratches.

I think that is a bit optimistic for a credit card sized one. I prefer a high energy ferrocerium spark - more use in the UK

You may find the moire patterns as the two sets of concentric circles interact rather distracting. If you can hold them apart by the right amount ~0.7f you can make a sort of Ramsden eyepiece:

I had cataract surgery and decided to be nearsighted, since most of my life is close-up.

But Mo is getting farsighted and can't read tiny labels at the grocery store. She sometimes buys whole wheat pizza dough and bread (yuk) instead of proper white sourdough. I figure that she can keep one of these in her wallet.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Yes, but you would need a meter or 2 for full absorbtion absorbtion of EM radiation in ice is the same So a few mm lens should be no problem, but I did not try it myself.

They work fine. SWMBO forgot her lighter and wanted to smoke. I have such a lens in my purse and lighted the cigarette.

Arie.

I could (and many I am sure) just use my glasses.

I thought the water heat-absorbers on cinema arc lamps were only an inch or two thick.

I started carrying a cell phone just as a timepiece (cuz every time I asked someone "what time is it?" they looked at their cell phone -- hey, I can do that!).

The *second* application I found for it was as a camera... so I could snap a photo of some tiny thing and then enlarge it (zoom) to see the fine detail. This is especially handy for reading the microprint that is now common on devices (does that say "12V @ 0.2A" or "15V @ 0.2A"?). The flash/illuminator makes this particularly useful in low light situations (like when you are trying to examine a device at the bottom of a pallet of other devices and don't want to have to unload the pallet just to gain access to it!)

I used to keep a jewelers loupe in the console of the car but that proved too tedious (trek out to the car to fetch it any time you needed to see something "fine" -- the phone was in your pocket as a timepiece so much handier!)

My Samsung phone makes an excellent magnifier, about 4x.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Ice is much more transparent than water in the near infra-red region. Both are very transparent in the visible part of the spectrum. Most of the energy in sunlight is in the visible and near infra-red. John

Not mine. They are concave.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

As long as it can create a real image of the sun, it can ignite e.g. a cotton lint. Fresnel lenses are no different from glass lenses in this respect. As a survival tool a Fresnel lens has the advantage that it is much lighter, especially compared to larger glass lenses.

FCS, it is a normal lens. I have one, and it is light!

The main thing is the surface area, that account for the power collected. Getting the image of the sun smaller, increases the temperature but that is not that important.

You realize that a Fresnel is thin,irrespective of focal length? If you want to make the focus length smaller, you just have to redesign the lens. No need to stack.

Having dark absorber material makes a big difference. White cotton wool is much harder to ignite than something that has already been charred once.

It gets difficult for a Fresnel lens to be anything faster than f 0.2 because the ridges start to interfere with the light path to the outer edges of the aperture. Aberrations also get really bad at fast f ratios.

Flat OHP condenser lenses are a good source of cheap big fast ones.

For the purpose of emergency fire starting, half decent lenses are okay.

There is a guy publishing experiments with the optics of a flat tv screen on youtube.

Groetjes Albert

Do you mean from the venerable Sinclair TV80 flat screen portable TV?

One of my friends won one on the first ever edition of Krypton Factor.

There are pretty decent polarizer sheets in LCD screens. Using these, I discovered that certain Chinese brands of polyester rulers make fair quarter-wave plates. Build your own quantum optics experiments for free!

Jeroen Belleman

One of the cuter physics demos with polarisers is to create an image in clear Sellotape on an OHP slide and then cross them to do the reveal.

You have to calibrate your choice of tape and number of layers.

I used a child's drawing of a house.

In the bad old days long before thin film deposition they used to make research grade H-alpha filters out of a stack of layers optically clear calcite crystals and highly polished optically flat quartz wave plates.

They were very difficult to make and horrendously expensive!