Naked Science Forum
Non Life Sciences => Technology => Topic started by: fizzfez on 23/05/2010 01:20:05
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Can anyone describe the smallest array of lenses and/or mirrors that would be needed to accomplish this?
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A little research on the web shows the relative energy per unit area from a bright, full moon, to direct sunlight to be between 1/100,000 to 1/1,000,000. If you can start a fire in bright sunlight with (say) a lens of area 5 sq cm then you would need between 100,000 and 1,000,000 such lenses. The total area would be between 50 and 500 sq m. With such a large lens a practical type to use would be a Fresnel lens.
However I would recommend use of a match.
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The Photon Energy Transformer & Astrophysics Laboratory (PETAL) at Ben Gurion University is the largest parabolic solar dish - it can concentrate sunlight upto 10000 times. Using Graham's most favourable figure of moonlight being 100,000 weaker than sunlight, then the PETAL could presumably magnify moon energy to about one tenth of normal sunlight. Warm you up a bit perhaps and read a book - but not start a fire. http://cmsprod.bgu.ac.il/Eng/Units/bidr/Departments/EnvironmentalResearch/solarcenter/Facilities/PETAL.htm
I suppose you could place loads of small flat mirrors in the spherical dish at the Arecibo Observatory - the dish there is 150m radius. Not sure how much it would focus (ie not parabolic but spherical) but with 70,000m^2 not certain that would matter. And I think Cornell might be a little peeved if you screwed up their telescope. http://www.naic.edu/public/descrip_eng.htm
The largest solar reflecting arrays are use fresnel reflecting or parabolic trough mirrors and have easy enough square metres (the biggest are nearly 500m^2) but they do not concentrate onto a single point but onto a pipe. http://solareis.anl.gov/documents/docs/NREL_CSP_3.pdf
So no lighting a fire with any of these. You will have to buy a couple of hundred thousand mirrors and mount them on a very flat surface in a fresnel reflecting array. That would mean the mirrors in the centre are flat and as you move out the angle increases. you would need a greater acreage of mirrors than a perfect parabolic dish as the focus is very vague. i think your financial backers might ask why you are insisting on using the moon as power source [:)]
Matthew
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I am very confused b.cos I am unable to understand so I want to know about it
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Although I'm happy to accept graham.d's figures, I don't think it's enough to just specify the collecting area, in this case 5 sq cm, of the lens because it doesn't tell us into how small an area the collected light is concentrated. A shorter focal length lens, with the same area, will concentrate the same amount of light into a smaller area, which will increase the intensity at the focus.
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Lee, I don't think "setting fire to something" is a very precise definition. I'm just going along with the practical idea of setting fire to straw from my youth that I remember. In a safe way, I hasten to add. You could argue about this if you want but really this is a rough order of magnitude calculation.
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There's a practical limit to lenses that makes it rather difficult to get the focal length less than the diameter.
if you assume that you have a lens that's pretty good then the abillity to light things is pretty closely related to the area of the lens.
Incidentally, I gather that there's a "spare" Hubble telescope mirror somewhere. Do you thing we could borrow it? It's only about 5 m2 so I don't think you could sart a fire with it using moonlight unless you were a lot closer to the moon.
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Hey, I wasn't disagreeing, just pointing out that there was another factor that had to be taken into consideration [:)]