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

[John Mallinckrodt <ajm@csupomona.edu>]

On Apr 16, 2006, at 10:27 AM, jbellina wrote:

> 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.

I disagree that my comment "misses the point."  Indeed, I said  
explicitly that it would probably be perverse to assume that one of  
the things that is equal is the f/stop.  The reason I said that was  
entirely to avoid missing "the point of the original question."

But I think you may have have missed my point, which was intended as  
an amplification of John Denker's original point about the "perils of  
OTBE".)  There is, in general, no way that ALL other things can EVER  
"be equal."  The fact that the original question only specified focal  
length in no way suggests that the questioner intended the apertures  
also to be equal since that would require that the f/stops (or,  
equivalently, the angle of convergence of the ray bundle) NOT be  
equal.  There is no way to avoid making what is ultimately an  
unwarranted (even if probably correct) assumption that the questioner  
intended for the apertures to be equal.

Notice that this is not the same kind of thing as deciding to neglect  
air resistance when a problem statement says nothing about  the  
issue.  We can equally well answer the fire starter question under a  
number of different assumptions and the answers we would obtain would  
be different.  Thus, the question is fundamentally underspecified.

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>