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

[Bernard Cleyet <bernardcleyet@redshift.com>]

"I wanted to add one more piece of information that will impinge on  
the answer here, and Ludwik 

[You mean bc?]

got this one just backwards. If one  
wishes to minimize spherical aberration in this application (assuming  
aspheric lenses are not among the unspecified "inequalities" in this  
problem), he should orient a plano-convex lens with its curved side  
toward the Sun, not its flat side. That is a much more sophisticated  
inequality that, I suppose, must be mentioned by the ETS to avoid  
legal bills. [I see Bernard Cleyet picked up these same points.]"

-----------

I remembered Nussbaum and Phillips writing min. SA occurs when the refraction is least and found J & W write when they are equal.  either are intuitive, i.e. the less one does to the ray the less opportunity to introduce aberration.  

I had this correct, but the application wrong for about 50 years.  Yes flat side towards the spot is better.


Best is w/ double non equi-convex and the side towards the spot nearly plane.  

As others have mentioned - great thought problem. But not for SAT.

Using my GE radiation Calculator I find crown glass absorbs a not insignificant amount of the incident sun's energy, i.e. about 13%.

bc



Leigh Palmer wrote:

> On 17-Apr-06 Ludwik wrote:
>
>  
>
> >
cut

> > > Leigh
> > >      
>
> I guess my exposition is rusty, Ludwik. I think I said exactly what  
> you said I missed about absorption in the lens. At some absorptivity,  
> given any fixed lens geometry, there must be a crossover from "short"  
>
>  
cut

> I wanted to add one more piece of information that will impinge on  
> the answer here, and Ludwik got this one just backwards. If one  
> wishes to minimize spherical aberration in this application (assuming  
> aspheric lenses are not among the unspecified "inequalities" in this  
> problem), he should orient a plano-convex lens with its curved side  
> toward the Sun, not its flat side. That is a much more sophisticated  
> inequality that, I suppose, must be mentioned by the ETS to avoid  
> legal bills. [I see Bernard Cleyet picked up these same points.]
>
> So we see that Larry's simple question (in my opinion a very good  
> question) leads to just the sort of discussion that we as teachers  
> need to nurture among our students or, failing that, between  
> ourselves and our students. I have only just scratched the surface of  
> the discussion here. We still have to work on the fire starting  
> problem at the receiving end of the radiation.
>
> Leigh
>
>