
Random Destructive Acts via Focused Solar Radiation - fezz
http://bclee.net/lens.html
======
awakeasleep
In case anyone reads this and gets excited enough to buy one, realize fresnel
lenses are actually dangerous.

It's almost like keeping some turpentine soaked rags in a cardboard box. Sort
of kidding. The point is, you can have the lense somewhere you think is safe,
but the angle of the sun changes throughout the year and one day six months
later rays of light hit it. Or you have kids that don't think that way, and
they leave it somewhere.

Even as a fully aware adult, it's counterintuitive, because you'll be trying
to take it outside and accidentally burn yourself, or set it agains something
while you're getting ready, not realizing the lens was born ready and doesn't
care about your plans. You always have to carry them in a sheathing material.
Your brain expects something like that to have an off switch.

They're cool, but you should probably keep them in an opaque sheath in a gun
safe, and leave it alone when you're drinking.

~~~
trhway
if only somebody gave the same advice to the "death ray" buildings architect -
Vinoly.

> but the angle of the sun changes throughout the year and one day six months
> later rays of light hit it.

almost exactly word for word :)

[http://www.theguardian.com/artanddesign/2013/sep/06/walkie-t...](http://www.theguardian.com/artanddesign/2013/sep/06/walkie-
talkie-architect-predicted-reflection-sun-rays)

"The developers have blamed the problem on "the current elevation of the sun
in the sky," a position Viñoly seems inclined to share.

"When I first came to London years ago, it wasn't like this," he said. "Now
you have all these sunny days. So you should blame this thing on global
warming too, right?""

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SamReidHughes
The Explorabook (
[http://www.amazon.com/dp/1591747996](http://www.amazon.com/dp/1591747996) )
came with a Fresnel lens as one of its pages.

Yes, a _children 's book_ came with a _Fresnel lens_ inside!

A kid detached his and brought it to summer camp, and after a few days, he was
asked to leave it at home.

~~~
TeMPOraL
> _A kid detached his and brought it to summer camp, and after a few days, he
> was asked to leave it at home._

That's how we kill creativity and curiosity in people.

~~~
SamReidHughes
It was just some stupid kid that went around cooking human flesh and being a
general nuisance.

~~~
fit2rule
What a pity the opportunity to foster an interest in science and technology
was not taken. Surely there would have been some value in integrating the
interest with the camp activities .. unless of course, thats not the purpose
of camp.

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ntumlin
There's a lot of videos on YouTube of people using Fresnel lenses from old
projection TV's and melting various things.

[https://www.youtube.com/watch?v=jrje73EyKag](https://www.youtube.com/watch?v=jrje73EyKag)

~~~
ChuckMcM
That was where I got mine, I asked at a TV repair shop if they had one, and
they gave me two! Setting the asphalt on fire was entertaining. We built a
simple frame for it out of frame components at the craft store to make it
easier to point.

My goal was to build an aluminum can smelter. At the time I had this idea that
I could crush a diet Dr. Pepper can with my hands (this I _often_ do) and then
slide it down into a crucible at the focal point of the lens, and have it melt
and drip into a bucket of water below, creating aluminum bb's which would be
much denser than even crushed cans. The problem however was that aluminum
oxidizes before it melts, so in the ideal set up you really want it running in
an oxygen free environment. Way more work than I wanted to invest.

Later I discovered induction heating (which you can do with a windmill, some
magnets and a coil of wire around a piece of steel (or aluminum).

~~~
nsajko
About the oxidation, have you tried using a reducing agent (Na is probably
easiest to get).

Did you watch those videos of guys melting aluminum in their backyards? :)

~~~
ChuckMcM
I did not try sodium. But to be clear, the picture of a place to throw my
empty soda cans which turned them into melted bits was the attraction, all
without hooking up to grid power. I'm not actually trying to smelt aluminum as
some are.

But watching folks melting alumninum is instructive.

------
jonah
Markus Kayser made a "Solar Sinter" e.g. a sun-powered 3D printer.

[http://www.markuskayser.com/work/solarsinter/](http://www.markuskayser.com/work/solarsinter/)

~~~
TeMPOraL
That uses _sand_ as printing material. I.e. you go with it to a desert and
start reshaping the environment! Absolutely amazing!

~~~
Gravityloss
They're thinking of doing something similar on the moon. Or why not on
asteroids or elsewhere? You can create dust free landing pads, tiles for
building, roads... Maybe just harden a regolith berm surface to lessen dust
problems.

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danieltillett
When I was a kid I used to have one of these huge lenses (it was made of glass
and made to make your 12” TV look like it was 30”). I used to use it to go
around setting various things on fire when I was being bad and kill weeds in
the lawn when I was being good. It was amazing how quickly it would work - a
few seconds and the weed was dead.

------
salgernon
Haltec in Mountain View (not to be confused with Halted in Sunnyvale) used to
have one on display; it was about a meter width I seem to recall. There was a
sign on it: "Warning: Sets Fires Instantly".

(Haltec is gone now, but I seem to recall some serious lasers still available
at Halted.)

