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



I'm sorry, but I just can't figure out what Jack and Bernard are saying because it doesn't make any sense with respect to what I observe experimentally and what I observe when I do ray tracing.

I suggest the following two things.

(1) Get a long lens 500 to 1000 mm focal length. Take it outdoors in the sunlight and see how small of a spot you can get on a piece of paper. The smallest spot you can get will occur when the sun's image is in focus. Any out-of-focus image will be a larger spot than the in-focus image.

The smallest spot will be sin(32.5')*(focal length) where 32.5' is the approximate angular diameter of the sun. Since the sun is very far away, its in-focus image will be at the focal length of the lens.

I challenge you to get a smaller spot than the spot you obtain when you have a sharp image of the sun projected on the paper.

(2) Click on the link below to get a PDF file of a drawing I made.

www.bluffton.edu/~edmistonm/sun_lens_image.pdf

(there are underscores between the words sun_lens_image)

The red rays from the top of the sun come to focus at the focal plane as shown. The blue rays from the bottom of the sun come to focus at the focal plane as shown. The spot at the focal plane where the sun's image is in sharp focus is the smallest spot you can get.

This drawing was made using regular ray-tracing rules for a thin lens. To fit everything on the paper, the sun is too close to the lens. The primary thing that changes when the sun moves further away (to the left) is the focal plane on the right moves toward the focus (F). At 93-million miles away the sun's image (i.e. the focal plane) will be at the focus of the lens for all practical purposes. But the spot will not be a point because the sun is not a point. It's angular diameter is about 32.5 arcminutes. The rays through the center of the lens will be diverging at 32.5 arcminutes. When these rays have reached the focal plane they will have spread to a circle of diameter sin(32.5')*(focal length) and that is how you can calculate the minimum diameter of the spot.


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