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Hi all-Scout
I only got into this because, contrary to my memories of Boy
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."burning
The reality is, amusingly enough if you check the internet,
glasses" in the 19th century -commonly used by chemists- were usuallylens
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
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-convexto
lens.
So unless my memories are totally distorted Mike should be able
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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Forum for Physics Educators
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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
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