Half a croissant, on a plate, with a sign in front of it saying '50c'
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Wok into the light.

Look ma. No mirrors. The closest I could find searching was a solar box oven, but I think this is different enough to post.
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(+2, -1)
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Two parabolic glass lids fit snuggly within a wok base for transporting, but when fastened together, underside to underside, they become a magnifying lens with a focal point the exact distance to the inner black surface of the wok.
When sitting on an insulated base pot, enough heat should be captured to boil water. The insulated base then becomes a warming oven.

mirage hologram maker http://www.optigone.com/
OK, nothing to do with solar heating, but another interesting use of two parabolic mirrors. [xaviergisz, Sep 01 2009]

Fly_20eye_20water_20lens [2 fries shy of a happy meal, Sep 01 2009]

Lots of different solar cookers http://solarcooking.org/plans/
Even a fresnel one and the mentioned box cookers - but nothing using a lens like the one described here. [jutta, Sep 21 2009]


       A 14" diameter lid would have only so much light falling upon it in broad daylight; I haven't crunched the numbers but from gut feel I doubt it's enough heat to provide decent cooking.
RayfordSteele, Aug 31 2009

       1. Lenses don't tend to be parabolic (not that that's very important, as you don't need a very precise focus)   

       2. Hollow lenses don't really work. If you filled the lens with water, which could work if the mating surfaces were precisely ground, then it would work. Or use a Fresnel lens.   

       3. Unless the sun is directly overhead, the water will not stay in the same place as the focussed light.   

       4. Wait, are your glass lids fastened together like this (), or like this )(?
spidermother, Sep 01 2009

       Like this: (). I suppose they could be water filled if the upper one had a depression so as to not make an air bubble.
I had never thought of a Fresnel lens.

       even assuming a rather big Wok, it would not catch all of the 100-300W/m2 incident solar radiation, so you'd try to cook with 50-150W, which is about the amount of energy produced by 1-3 small candles... a very slow simmering, but nothing you'd normally use a Wok for.
loonquawl, Sep 01 2009

       If the lens fits inside the wok, it'll capture as much sunlight ,exactly, as the wok would, sitting out in the sun by itself. Focusing it won't increase the net amount of heat, just concentrate it in a very small area. So you'll end up with one blackened water chestnut, and a wokfull of warmish, raw, slightly suspect stir fry. No thanks.
BunsenHoneydew, Sep 01 2009

       I just wanted it for boiling water while camping. I thought that the shape of the wok would be the focal point no matter what angle the sun was at. Water would be the only thing you would heat this way because the light needs to be able to go through it to work.
I like the idea of many lenses UnaBubba but bungston sort of beat me to the punch on that one. [link]


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