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

[jbellina <jbellina@saintmarys.edu>]

I can understand why the diameter of the lens in important, the  
larger the area, the more energy focussed, but I don't really  
understand why the focal length would play such a large role.  It  
certainly is part of the f-stop formulation, but I don't see how it  
could have a large impact on fire starting.  Care to explain?

cheers,

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On Apr 16, 2006, at 8:53 AM, John Denker wrote:

> Larry Smith wrote:
> > All other things being equal, would a lens with a short focal  
> > length or a
> > long focal length be better as a fire starter?
>
> The question is underspecified.
>
> The brightness (and therefore to a first approximation, the  
> temperature) of
> the spot depends on the f/stop (aka numerical aperture) of the  
> lens.  This
> depends directly on the diameter and inversely on the focal length.
>
>   ==> For any given diameter, a shorter focal length is better.
>
> However, if the spot is too small, heat will be lost to conduction  
> etc. so
> fast that you'll have problems.  For any given brightness, a larger  
> spot is
> better.
>
>   ==> For any given numerical aperture, a longer focal length is  
> better.
>
>
> --------------
>
> This is yet another illustrations of the perils of OTBE ("Other  
> Things Being
> Equal").  The problem is, it is often hard to guess *which* other  
> things, if
> any, are being held equal.
>
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