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Addonex

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I thought this was pretty cool, and demonstrates the concept of a focal point pretty brilliantly.

Jem melts rock using sunshine. [VIDEO]

"but can it pop balloons?" :crackup:
 

BShanahan14rulz

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you take a lens far enough from the source of light, you can get a beam divergence approaching zero, as the apparent size of the light source gets smaller and smaller, approaching zero.

I skipped through the video and saw the part where they were melting an orange piece of plastic set on top of a bolt.


Eh, that's not so great.






Rewatched it and it turns out that wasn't molten plastic, but rather molten steel.
 

jbtm

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Video looks fake to me, though the concept is certainly sound.
Concept is fully. I've seen people with large magnifiers who can light a shoe on fire instantly, nothign new. I more wonder how they are gettin the sun light into that room, specifically, on that table. They would obviously have to have some large lens outside that can track the sun so it would always be light, and then beam it down to that room, and expand it to form that X zero-point focal.
 

wbp

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bbshamsa

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Video looks fake to me, though the concept is certainly sound.
Looks real to me.

It's a BBC production anyway, so very unlikely to be inaccurate.

Presumably, you know how much energy is in 3m2 of sunlight, and how to calculate the increase at the focal length shown?
 
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Benm

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I think its real - and entirely feasible too. I suppose the object on top is a mirror, and it collects light from the sun using other flat mirrors that track the position of the sun (heliostats or whatever you want to call them).

The amount of power collected by 2 m2 depends on location, season and time of day. It can be up to about 2.6 kilowatts, but if this is in brittain and not extactly at noon, i reckon its in the order of 1 kilowatt.

You could built this installation yourself, and it wouldnt have to be overly expensive either. The problem is obviously getting a 2 square meter mirror or lens. Buying one of telescope quality will be very expensive, but if you settle for a slightly less defined focal point, there are several options.

The most cost effective would be a fresnell lens. That could be sourced from a discarded lighthouse, but they are also manufacturerd for this purpose. Look at this for example: HUGE 43x33 FRESNEL LENS HOT WATER SOLAR OVEN HEATER SUN - eBay (item 180589562064 end time Dec-18-10 15:13:11 PST)

Its 127 bucks, and has almost 1 square meter collecting area.

The even sell a sizeable mirror for $300 or so: http://www.youtube.com/watch?v=Ij1YvJT5tLQ
Collecting area is somewhat smaller (its ~90 cm diameter), but probably optically superior to a fresnel lens.

And if you want it really cheap: Find a bigass discarded sattelite dish, and apply tinfoil. Optical quality will suck, but plenty of power to go round :D
 
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bbshamsa

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*rolls eyes...


... To all you nay-sayers who seem so confident that if YOU can't figure out it's feasibility, it therefore cannot possibly be true, here's a link to the Solar Furnace Research Facility in southern France where this video was recorded.

Four solaire d'Odeillo - Wikipédia

It's in French of course, so if the pretty pictures aren't enough to help you get it, try and paste some of the text into Google translator or something.
 

Benm

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Wow.. thats a lot bigger than 2 square meters :D

Pity the fool who has to clean all those mirrors!
 

IsaacT

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hmm....lasers are already an expensive hobby for me!!! Now I want to go buy a bigass lens so I can melt steel!
 

bbshamsa

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Wow.. thats a lot bigger than 2 square meters :D

Pity the fool who has to clean all those mirrors!
Yeah, that bit is a bit unclear in the sense they clearly say the energy in that demo was from 2 square meters of mirror, but don't show precisely how.

I mean we could assume that as the mirrors can all be controlled independently they could have set it up to only allow 2 square meters, cross-section of rays, non-focused, to reach the mirror above the demo area.

I remember watching the whole piece on the show and I'm pretty sure I remember them saying that the energy from all the mirrors is a LOT more than shown in that demo with the rock. and they had to go to a special section of the test facility to show it working at full power.

In the full power demo they used all the mirrors and the test piece just liquified immediately to a water like consistency.

The steel bolt in that demo melted a lot more slowly, and nowhere near as much.
 




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