Re: [Phys-l] Phys-l Digest, Vol 15, Issue 15

[Leigh Palmer <palmer@sfu.ca>]

On 16-Apr-06 Larry Smith asked:

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

All other things being equal, the concentration of radiant energy is  
greater for a short focal length lens than it is for a long focal  
length lens. The question is entirely reasonable and, I think, well  
specified.

One would have to contrive a situation in which the correct answer to  
this question is not that a shorter focal length lens is a better  
fire starter. The only one that comes to my mind is an utterly  
unrealistic one in which the lens material is lossy. In that case a  
thicker (shorter focal length) lens will absorb more energy than a  
thinner lens.

Leigh