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.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu