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





"True, in some cases the sun's image may be smaller than the diffraction spot, but I'm ..."


Now am am confused. An image can be smaller than the diffraction spot?


"'Focussing the sun' is, in principle, the wrong action to focus upon (ha!) if you want the smallest spot."

Right, the focal point defined by the coaxial to the PA rays inside of the paraxial zone is not the location of the smallest spot (circle of least confusion). Were discussing simple magnifiers, yes?

bc, missing something?

Jack Uretsky wrote:

See the crucial comment below.


On Tue, 18 Apr 2006, Michael Edmiston wrote:


Jack said, "The correct question is, how small a spot can you make? The
applicable physics is related to diffraction."

Stefan said, "Just a comment... If I recall my fourier optics class
correctly, no real lens system can direct all "rays" to the focal point,
even in the case of perfect planes waves emanating from a source an
infinite distance away (for which the sun mostly qualifies)."

I agree with Jack's first sentence. Getting all the sunlight entering
the lens to hit the fuel within the smallest spot is the goal. So the
question is how small of a spot can you get from the sun, and/or how do
you achieve the smallest spot from the sun?

I disagree with the Jack's second statement about the diffraction limit.
Stefan's statements also appear to invoke the diffraction limit. In
addition, Stefan indicates the sun mostly qualifies as a source from an
infinite distance. These ideas are not correct for answering the
question of how small of a spot can you get from a lens that is focusing
the sun.


That was my point. "Focussing the sun" is, in principle, the wrong action to focus upon (ha!) if you want the smallest spot. True, in some cases the sun's image may be smaller than the diffraction spot, but I'm
concerned with the principle involved. If the sun's image is not diffraction limited, then "focussing the sun" is inconsistent with getting the smallest (and hottest) spot.


It is indeed true that diffraction places a limit on the resolution of
the lens. By this we mean that light leaving a point on the object does
not all arrive at the same point in the image. Indeed we do discuss the
diffraction limit for optics, especially telescope optics where the
diffraction limit keeps us from imaging a star as a point, and it keeps
us from imaging the individual stars in clusters or galaxies that are
too far away.

As stars go, our sun is an exception. Its distance does not qualify as
infinite because its diameter is too big compared to its distance.
Rayleigh's criterion for resolving images is that the angular separation
must be greater than inversesine(1.22lambda over lensdiameter). For
example, with a 90-mm diameter lens, the Raleigh diffraction limit for
550 nm light is about 4.27E-4 degrees or 2.56E-2 arcminutes or 1.54
arcseconds.


Regards,
Jack>