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Re: [Phys-l] fire starter from the sun - revisited



right idea wrong approach -- aim it at the people, especially the helmsman.

bc

p.s. Just returned from workshop, wherein I used a large shaving mirror to show X-band microwaves differ (basically) little from the vis. While carrying it and a large wax lens up to the lab. my face was heated so quickly and much I almost dropped them. Just before doing the xpt. we went outside to do a crude bench determination of its F. It's an f/ 1 w/ F~= 40 cm. The image projected on a sheet of paper, tho large, about 5 cm d., quickly charred the paper. The predicted size should be ~ < 1/2 cm (correct? Dr. E.) that I found it 10 X is either the crudeness of the measurement (one held a meter stick another the mirror and the third the piece of paper), or the extreme spherical aberration. My conclusion is, similarly to the discussed lenses, forget about diffraction.

Polvani, Donald G. wrote:
The "Myth Busters" TV show had a segment several months ago
investigation the myth of Archimedes' "Death Ray". According to the
ancient myth, Archimedes had invented a means of harnessing the sun's
energy to destroy enemy galleys by setting them on fire. The TV
producers actually got a MIT professor and some of his graduate students
to try and test the myth. The MIT group built a series of polished
metal mirrors (I believe there were something like 50 mirrors and they
were flat so that the sun's image from many mirrors could be easily
superimposed given the unknown range of the galley). They ran a series
of tests with a wooden barge towed past the bank of mirrors (after first
running a calibration run where the mirrors were adjusted to produce a
common image of the sun on the path of the barge). They did succeed in
creating smoke and burning a little of the wood on the barge (which had
been treated with pitch as it might have been in ancient times), but
never succeeded in really getting a good fire going. The conclusion was
that Archimedes "death ray" wasn't exactly a showstopper when Roman
galleys invaded Syracuse.

Don Polvani
Northrop Grumman Corp.
Oceanic and Naval Systems
Annapolis, MD

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of jbellina
Sent: Monday, April 24, 2006 2:59 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] fire starter from the sun - revisited

You just reminded me. I recall there being a very large mirror assembly
in France...perhaps meters across with many mirrors which focus the suns
rays on one spot for melting metals.

cheers,

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On Apr 24, 2006, at 2:48 PM, Jack Uretsky wrote:


Hi all-
I only got into this because, contrary to my memories of Boy

Scout
days, someone - maybe it was Mike - seemed to suggest that you can't start a fire with a magnifier. The logic is, the smaller the spot then the hotter you can get it. The smallest possible spot size is given by the diffraction limit. Now, it seems, Mike and Brian want to



challenge this logic with reality.
The reality is, amusingly enough if you check the internet,

"burning
glasses" in the 19th century -commonly used by chemists- were usually mirrors. The Brits even managed to set fire to a ship model by focussing a bunch of burning mirrors on them.
Mike's challenge involves long focual-length lenses and thin

lens
formulas. But the old burning lenses of my carefree youth were thick plano-convex flashlight lenses. So there are two corrections needed for Mike's experiment. The lens diameter is the diameter of a coherent bundle of rays from the center. "Coherent", here is wave language, somewhat inconsistent with "ray language. In a cheap lens this is probably much smaller than the physical diameter of the lens.



Secondly, the focal length is very short for a thick plano-convex lens.
So unless my memories are totally distorted Mike should be able

to
raise a blister on the back of his hand with a thick, old-fashioned, plano-convex flashlight lens. I have no opinion on whether or not the



hot spot will be diffraction limited.
Regards,
Jack




On Sun, 23 Apr 2006, Brian Whatcott wrote:


At 11:31 AM 4/17/2006, Mike Edmiston, you wrote:


Jack Uretsky said, "We're not focussing [sic] the sun's image, we are placing the focal point of the rays from the sun on the fuel."

I repeat: a lot of people don't have experience playing with long focal length lenses. I suggest you quit using paper and pencil and grab a long lens and go out into the sun. I just did that to assure



I was remembering it correctly.

Jack's post seems to imply it is possible to get the bright spot of light to fall inside a smaller circle than the circle formed when the sun's image is in focus. This simply is not true. When "focusing" the sun's rays with a positive lens, the smallest spot occurs when the sun is in focus. The sun is not a point source and cannot be focused to a point. With a short focal length lens it may



look like you are getting a point, but with a longer lens it is obvious you are not getting a point.

I just tried a 90-mm diameter telescope lens with 1000-mm FL (f/ 11). It is not even capable of burning my hand, although it does get warm.
Starting a fire is not a remote possibility. Without measuring, I estimated the smallest spot (which occured at image focus) was about



10 mm.

I also tried a 12.5-inch diameter telescope mirror with 5-ft FL (f/5).
As with the 90-mm lens, you cannot get a spot. The smallest circle I could get was the one that was in focus, and I estimated it was
12 mm in
diameter. In this case the concentration was sufficient to burn paper (although not instantly), and a fire probably could have been started (but my paper did not ignite).


Michael D. Edmiston, Ph.D.

Here are two observational data points at least.
Taking the well-known Figure of Merit for burning glasses
- where higher equals better, given by lens diameter squared divided by focal length, we have
1) A telescope lens: 90 X 90 / 1000 = 8.1 f.o.m and
2) A telescope mirror: 317.5 X 317.5 / 1524 = 66.3 f.o.m

One might conclude that the mirror was eight times more effective, but this simplified model excludes considerations of absorption in the reflecting and refracting glasses, where the reflector is usually



lossier.

A magnifying glass of 3 inch (76mm) diameter and focal length 3 inches (76mm) would be assessed at f.o.m = 76, and would be a good candidate (as mentioned elsewhere)

It might be interesting to try two other candidates:
3) A clear glass bulls-eye marble
4) A round bulb glass vase filled with water.


Brian Whatcott Altus OK Eureka!


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--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley




_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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