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Re: [Phys-l] paraxial approximation




On Apr 10, 2008, at 2:09 PM, LaMontagne, Bob wrote:

Do most of you really consider ray tracing to be "physics"? The essence
of the functioning of an optical device seems to be to get the light
emitted from a point source and travelling through all possible paths
through the device to all arrive at some other point in space at the
same time. Ray tracing is a good aid to finding the point where the
image for a point object may appear, but it doesn't really seem to
demand that the light following various paths has to interfere
constructively at the image - at least not explicitly. In other words,
in the usual way it is implemented, ray tracing gives a necessary
condition for an image to appear, but not a sufficient condition. I
realize that it is possible to demonstrate that the paths indicated by
the rays are isotemporal, but I have never seen a textbook that
emphasizes that point (except through a mention in a few texts that
Fermat's principle leads to the law of equal incidence and relection
angles for a plane mirror.)

Bob at PC
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