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08-17-2008, 09:09 AM #1
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caleb
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Sun + Magnifying Glass = ?mw

seeing as i can light a fire with a magnifying. and the focused dot burns instantly at 5mm and burns takes a few secconds to get smoking at about 10mm. (measurements in diametre)
so could someone get a magnyfying glass and see how many mw there are in that dot. would it be a reliable reading.
maybe try it with different sized glasses, my magnifier is 10cm (diametre)

another thing i thought id throw in, when burning with a glass, is it safe to look at the dot?

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08-17-2008, 09:35 AM #2
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Switch
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Re: Sun + Magnifying Glass = ?mw

Well, considering that the Sun's power density on the ground is 1kW/m[sup]2[/sup], you should be able to easily calculate what power there is in a 10cm circle(because that's what you have concentrated in that dot there:all the sun on the surface of the magnifying glass, minus reflections)

And it's probably unheathy to look at the dot, but in this case , I think it IS better to use sunglasses, or even better yet, a welding mask or a film from a floppy disk. :P

08-17-2008, 10:43 AM #3
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caleb
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Re: Sun + Magnifying Glass = ?mw

Quote:
 Originally Posted by Switch film from a floppy disk. :P
floppy disc, i am an astronomer an was going o buy some solar film, never got around to it though. and its not expensive.
but, how much or the light does a floppy disc block?
i wouldn't stare at the sun with it but im just interested...
8-)

08-17-2008, 12:38 PM #4
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lazerguy
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Re: Sun + Magnifying Glass = ?mw

My magnifying glass is about 5cm in diameter, and on a hot summer day it can easily set old leaves, newspapers etc. on fire. 8-) I would estimate it to be equivalent to about a watt of laser light, but I'm not sure. A 10cm circle should be a lot more than a watt.

If the suns power really is 1kw/square meter, it would be pretty easy to do the maths. Then, the power per square decimeter should be ten watts, and a 5cm square would be 2500mw. if you take a 5cm squares power multiplied with 3.14 (pi), you would get the power of a 10cm circle. 2500 multiplied with 3,14 is 7850. With a 10cm magnifying glass you would then obviously have 7850mw of sunlight to work with. But that is of course IF that statement about the suns power/ square meter is correct. Wouldn't that value depend on which time of the day it is, which time of the year it is, etc? How can that be a constant number? :-?

Now imagine the power of a 1 meter circular magnigying glass. Some serious power. 8-)

EDIT: I obviously made the maths a little wrong, now I have restored the numbers by making them ten times as big.

08-17-2008, 12:47 PM #5
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Re: Sun + Magnifying Glass = ?mw

Quote:
 Wouldn't that value depend on which time of the day it is, which time of the year it is, etc? How can that be a constant number?
It does depend on various factors and is not a constant number. The absolute maximum is about 1300 W/m2, which is valid when the sun is directly overhead (noon, on the equator march 21st) and there are no clouds.

If the sun is not directly overhead, you can still get close to the maximum because you can (and will have to) hold the magnifiying glass at an angle so it points towards the sun. As long as there is no atmospheric effect hindering you, you'd still be close. The path through the atmosphere is longer though, and in practice you will notice that it gets lets powerful when the sun is closer to the horizon.

08-17-2008, 01:31 PM #6
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Re: Sun + Magnifying Glass = ?mw

If you want to get large amounts of sunlight focused, a big mirror would be better than a massive magnifying lens

08-17-2008, 01:48 PM #7
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Re: Sun + Magnifying Glass = ?mw

Quote:
 Originally Posted by Murudai If you want to get large amounts of sunlight focused, a big mirror would be better than a massive magnifying lens
Or you could just use a fresnel lens. ;D I've been looking to get one but I haven't gotten around to it, it is great for anybody who likes to go outside and burn things with a magnifying glass because a fresnel lens is 100x cooler.
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08-17-2008, 02:38 PM #8
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Re: Sun + Magnifying Glass = ?mw

I got quite a different result. Let's start by converting m[sup]2[/sup] to cm[sup]2[/sup]

1m[sup]2[/sup] [ch8801]

1m[sup]2[/sup] x 100cm x 100cm [ch8801]
* ** * * **1m * * ** *1m

[s]1m[sup]2[/sup][/s] x 100cm x 100cm [ch8801]
* * * * * **[s]1m[/s] * * * * [s]1m[/s]

100cm x 100cm [ch8801]

10,000cm[sup]2[/sup]

Now the area of a 10cm magnifying glass:

A=[ch960]r[sup]2[/sup]

A[ch8776]3.14 x (5cm)[sup]2[/sup]

A[ch8776]78.5cm[sup]2[/sup]

Now a ratio:

power = power
*area * * * area

__1000W__ = ___X___ =
10,000cm[sup]2[/sup] * * * 78.5cm[sup]2[/sup]

X = 78.5cm[sup]2[/sup] x 1000W
* * * * * 10,000cm[sup]2[/sup]

X = 78.5[s]cm[sup]2[/sup][/s] x 1000W
* * * * * *10,000[s]cm[sup]2[/sup][/s]

X = (78,500 ÷ 10,000) W

X = 7.85W

(EDIT: math corrected)
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08-18-2008, 07:44 AM #9
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Re: Sun + Magnifying Glass = ?mw

i wana get some massive mirror thing and annoy people over 3km away.
does anyone know what something like this is called.

08-18-2008, 08:54 AM #10
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Re: Sun + Magnifying Glass = ?mw

Quote:
 Originally Posted by Cyparagon Now the area of a 10cm magnifying glass: A=[ch960]r[sup]2[/sup] A[ch8776]3.14 x (10cm)[sup]2[/sup] A[ch8776]314cm[sup]2[/sup]
Wanna try that part again? It's a 10cm diameter glass.

ALSO: If you're in the US, check out this site: http://www.srrb.noaa.gov/isis/isispick.html. Pick the location closest to you, and get a very good guess as to the solar irradiance you're seeing at a given time of the day on a given day. There are other resources for a lot of this same type of data, as well.

08-18-2008, 11:26 AM #11
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Re: Sun + Magnifying Glass = ?mw

So it would still be 15.7W....damn, I'm never gonna get close enough with my 4W handheld, though, I'm pretty sure I could focus it better and possibily get more irradiance :-/

As for the floppy disk, it looks safe enough, but I can't really tell how much IR or UV is going through.... could just test IR with a camera, and UV with flourescence :P

08-18-2008, 01:01 PM #12
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Re: Sun + Magnifying Glass = ?mw

you have a 4 watt.where is it from
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08-18-2008, 01:51 PM #13
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Re: Sun + Magnifying Glass = ?mw

Quote:
 Or you could just use a fresnel lens.
Sure thing - the ones found in overhead projectors are about 30x30 cm in size and quite suitable to light campfires or bbq's on a sunny day.

The power collected is up to 100W or so, but focus is not as good as with a proper magnifying glass. It's still plenty of energy though, go try it out sometime

Quote:
 i wana get some massive mirror thing and annoy people over 3km away. does anyone know what something like this is called.
A nuisanse?

A mercedes dealership about 2 km from here has this massive rotating logo on their roof. When sun angle is just wrong, it flashes in my window every rotation - irritating!

08-18-2008, 02:28 PM #14
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Re: Sun + Magnifying Glass = ?mw

Old rear-projection TVs have a lens the size of the screen... in... er, the screen. One can melt pennies with that type of power.

Quote:
 Originally Posted by pullbangdead Wanna try that part again? *It's a 10cm diameter glass.
Don't you correct me! radius and diameter ARE the same thing!

I had a few drinks shortly before writing that. 7.85 Watts it is

Quote:
 Originally Posted by Switch So it would still be 15.7W....damn, I'm never gonna get close enough with my 4W handheld, though, I'm pretty sure I could focus it better and possibily get more irradiance :-/
Oh, without a doubt.

Quote:
 Originally Posted by pullbangdead If you're in the US, check out this site: http://www.srrb.noaa.gov/isis/isispick.html. *
Interesting... my area has recently only peaked at 800W/m[sup]2[/sup]
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08-18-2008, 05:14 PM #15
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Re: Sun + Magnifying Glass = ?mw

Quote:
 Originally Posted by styropyro [quote author=Murudai link=1218964146/0#5 date=1218979895]If you want to get large amounts of sunlight focused, a big mirror would be better than a massive magnifying lens
Or you could just use a fresnel lens. ;D I've been looking to get one but I haven't gotten around to it, it is great for anybody who likes to go outside and burn things with a magnifying glass because a fresnel lens is 100x cooler.[/quote]

You can get them from OHPs ( Over head projectors ) thats where I got mines :P
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08-18-2008, 09:28 PM #16
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Re: Sun + Magnifying Glass = ?mw

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