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

[jbellina <jbellina@saintmarys.edu>]

Interesting but I think misses the point of the original question  
which only specified focal length, so I suspect the person asking  
wasn't thinking of constant convergence at all.  In fact, they might  
have been wondering whether convergence, another way a saying focal  
length, made a difference.

cheers

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

On Apr 16, 2006, at 10:34 AM, John Mallinckrodt wrote:

> On Apr 16, 2006, at 6:00 AM, 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?
>
> The point is that one could assume (probably perversely, but not
> insensibly) that one of the things that is "equal" is the f/stop.  In
> that case, the longer focal length lens would also have the greater
> aperture and collect an amount of power that is greater by a factor
> of the square of the ratio of the focal lengths.
>
> This is equivalent to assuming that the angle of convergence of the
> extreme rays in the converging bundle is one of the things that is
> "equal."
>
>
> John "Slo" Mallinckrodt
>
> Professor of Physics, Cal Poly Pomona
> <http://www.csupomona.edu/~ajm>
>
> and
>
> Lead Guitarist, Out-Laws of Physics
> <http://www.csupomona.edu/~hsleff/OoPs.html>
>
>
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l