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Question about the sun and magnifying glasses

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kake

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I know this isn't laser related, but i hope it's okay anyway..

1x1meter on the ground on a sunny day could get you 1000W if you had a effecient enough solar cell. If i use a 1x1meter magnifying glass, and focus it down to a little dot (just like you do if you are burning ting with smaller magnifiing glasses) would the dot you get be like a 1000W laser?

All good answers will be rewarded with cake.
 





L

likewhat

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The intensity of the spot, that is 1000W/area of spot, would be the same as a laser that had that same intensity. You would not have any of the same mechanisms present like you would in a laser spot though, such as coherence and monochromaticity.

In practice that would be tough to do because the different colors would focus at different places, you would need something to compensate for that such as what is done in achromatic lenses.

Or you could probably use a parabolic mirror an that might work.
 
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kake

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So in teory, i could make a 10x10meter parabolic mirror, and start cutting metal sheets? :p
What kinda mW would you get on a sunny day with a standard 10cm diameter magnifying glass?
 
L

likewhat

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well, a m^2 gets you 1kW so a 10 cm diameter lens gets you (.05)^2*Pi*1000=7.85 watts or 7850 mW
 
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Switch

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Yea I would guess thats pretty acurate as I was able to cut a floppy disc in half rather fast with a 5mm dot using a dusty 9cm lens :p

I wonder how expensive would a huge 1m in diameter lens be?One of those crappy fresnel lenses should be cheaper, and with aditional optics you could make a cool sunlight cutter :D
 
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turtlecannon

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If you need a giant lens, projection TVs have convex lenses the size of their screens. Mine is starting to conk out and when it finally goes I'll have a 60 inch lens.
 
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mliptack

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:-?

Umm that would make projection screens ungodly heavy... A projection screen is like a projector with a box built around it and a really wide angle lens. There is sometimes a piece of glass on the front, but its not a huge lens.

Unless you have some weird prototype model... ;)
 
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Switch

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A huge plastic fresnel would still be cheaper and lighter and could go bigger, and the crappy output could e corected with additional optics ;D
 
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darklandz

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i remeber literally setting some tissue on fire with the sun/magnifying glass trick - yet even a >400mw green laser wont put a single scratch on a white tissue?
 
L

likewhat

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well, 400 mW just isnt very much power in grand scheme of power. A microwave oven takes like 2 or 3 minutes to boil a cup of water and it is 1 kW.
 
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Switch

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darklandz said:
i remeber literally setting some tissue on fire with the sun/magnifying glass trick - yet even a >400mw green laser wont put a single scratch on a white tissue?
Click to expand...

On white kevlar maybe :D No but really, I bet it depends on the tissue too.I'm sure not any white tissue could withstand that.

There's this video on youtube of a huge industrial laser (CO[sub]2[/sub] I guess) that heats up coffee and tea pretty fast :D
 
Benm

Benm

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If you want a reasonably sized fresnel lens, try taking apart one of those oldfashioned overheat projectors... the kind where you put A4 transparents under to project. The lens on those is around 30x30 cm - probably useful to set something on fire.
 
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steve001

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Years ago I sandwiched a 12x12 inch plastic Fresnel lens between to slightly larger pieces of picture frame glass for rigidity then fill the gap with silicone caulk. It burns very quickly.
 
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philguy

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Hello, I am a link! <- think you're looking for that.

Also, don't count for more than 700W/m² - that will make your calculations a bit more realistic.

You'd get a 1000W light source - but not a 1000W laser. The light you get will be continuous spectrum, incoherent and far from having been lased (although I'd trade any laser for a light source generating light the way the sun does...).


Although there is a way to get lasing from that - what I've heard, some japanese foused sun beams into a laser rod of some material (was it something with magnesium? maybe.), that absorbed where the solar spectrum peaks, and were able to get 26W of laser light real laser light) out of the sun. So basically a "sun-pumped-solid-state-laser.
So yup, there is hope, but not that way.
 
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