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

[John Denker <jsd@av8n.com>]

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?

Conservation of energy suffices.

Energy scales like diameter of lens, squared.

Area of spot scales like focal length, squared.

Same energy spread over more area ==> less fire.