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

[Jack Uretsky <jlu@hep.anl.gov>]

On Sun, 16 Apr 2006, Michael Edmiston wrote:

> I don't understand the comment about f/stop clouding the issue.  It
> seems f/stop (i.e. f/ratio) is exactly what people need to understand
> (for this case and many other cases).
>
> big, i.e. the aperture, although big, is not big compared to the focal
> length.  The sun does not focus to a spot, but focuses to a fairly large
> circle more than an inch in diameter.  "Why won't it focus sharper?"
> they ask.  I point out that it is sharp.  If there are clouds, you can

Hi all-
	I must be missing something if the discussion so far is correct.

	Heere's my analysis:  We're not focussing the sun's image, we are 
placing the focal point of the rays from the sun on the fuel.  The rays 
focus to an angle of about lambda/D radians, at a distance f from the 
lens, where lambda is the average wavelength, D is the diameter of the 
lens, and f the focal distance of the lens. The spot radius is therefore:
		(lambda/D)f
and the area is the square of this quantity (time pi).  The energy 
concentrated is proportional to D^2, so the lens diameter cancels out, and 
the energy density in the spot is inversely proportional to f^2.  Short 
focal-length lenses (thick lenses) work better - which accords with my 
memories of childhood.
						Regards,
							Jack

--
"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