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



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?

The question is underspecified.

The brightness (and therefore to a first approximation, the temperature) of
the spot depends on the f/stop (aka numerical aperture) of the lens. This
depends directly on the diameter and inversely on the focal length.

==> For any given diameter, a shorter focal length is better.

However, if the spot is too small, heat will be lost to conduction etc. so
fast that you'll have problems. For any given brightness, a larger spot is
better.

==> For any given numerical aperture, a longer focal length is better.


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This is yet another illustrations of the perils of OTBE ("Other Things Being
Equal"). The problem is, it is often hard to guess *which* other things, if
any, are being held equal.