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

[Brian Whatcott <betwys1@sbcglobal.net>]

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!