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

[Ludwik Kowalski <kowalskil@mail.montclair.edu>]

On Apr 16, 2006, at 4:19 AM, Larry Smith wrote:

> All other things being equal, would a lens with a short focal length 
> or a
> long focal length be better as a fire starter?

In a vacuum this does not matter, provided "other things being equal" 
also refers to the area of the lens and to the area at which 
intercepted energy is concentrated. But air might absorb or scatter 
some radiation and for that reason a short f lens is be better, at 
least in principle. I am also assuming that in both cases the substance 
to ignite is a perfect absorber.

Ludwik Kowalski
Let the perfect not be the enemy of the good.