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

[Stefan Jeglinski <jeglin@4pi.com>]

> 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.

Just a comment... If I recall my fourier optics class correctly, no 
real lens system can direct all "rays" to the focal point, even in 
the case of perfect planes waves emanating from a source an infinite 
distance away (for which the sun mostly qualifies). There is always a 
convolution with the aperture of the optical system, which is finite 
for all real lens systems. For some subset of lens systems, it may 
-appear- that there is a point at f, as the idealized ray diagrams 
often show, but this is basically because it's small enough that the 
eye cannot distinguish.

I may be misremembering this analysis - can someone confirm or deny 
(not having my FO notes/text handy).


Stefan Jeglinski