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

[jbellina <jbellina@saintmarys.edu>]

Exactly my point, since area is the issue, why cloud the conversation  
by referring to f-stop.  It happens to have lens diameter in the  
formula, but it just introduces another factor which is not  
needed...Ockham perhaps is needed?

cheers


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

On Apr 16, 2006, at 11:12 AM, John Denker wrote:

> jbellina wrote:
> > 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?
>
> Conservation of energy suffices.
>
> Energy scales like diameter of lens, squared.
>
> Area of spot scales like focal length, squared.
>
> Same energy spread over more area ==> less fire.
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