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

[John Mallinckrodt <ajm@csupomona.edu>]

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>