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



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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just before leading them into the Little Big Horn Valley