Re: [Phys-l] optics - diffraction?

[Jack Uretsky <jlu@hep.anl.gov>]

Hi all-
	Paul's language ia reminiscent of that in Frank's optics, long 
used at MIT.  I happen to be in mthe hospital at the moment (for a bypass)
so can't be more precise.  But this is likely the text Feynman learned 
optics from.
				Regarda,
					Jack

On Tue, 28 Apr 2009, John Denker wrote:

> On 04/27/2009 03:02 PM, Paul Lulai wrote:
>
> > Each infinitesimal
> > point on the opening of the slit acted as a point source.
>
> OK, we have a _single_ slit consisting of _multiple_ points.
>
> > Early on, I remember learning that single point diffraction patterns
> > are (were?) the result of Huygen's principle.
>
> That phrasing strikes me as strange.  There is a huge chasm
> separating physical reality from theory.  The physical waves
> do what the physical waves do.  They aren't the "result" of
> any theory, from Huygens or anybody else.
>
> > The
> > interference pattern is (was?) the result of these multiple point
> > sources within a single opening.
>
> Again, I wouldn't have said "result of".  It would be better to
> say the interference pattern _could be explained in terms of_ a
> number of point sources.
>
> > I've also heard hand-waving arguments stating that Huygen's Principle
> > isn't correct.  This interference pattern is the result of something
> > else.
>
> Well, Huygens' principle does have some limitations.  It works
> qualitatively for coherent _forward_ scattering and doesn't work
> at all on the back side of the slit.
>
>  For details on this, see D.A.B. Miller,
>  "Huygens's wave propagation principle corrected"
>    http://www-ee.stanford.edu/~dabm/146.pdf
>
> For that matter, the notion of a structureless "slit" is already
> something of an idealization, so you need to worry about that, too.
>
> > What is it if not Huygen's multiple point sources interfering with
> > each other?
>
> Again, the physics is the physics.  You can't ask whether the
> physics "is" this-or-that theory or not.
>
> If the physics is not _well described_ by the Huygens construction,
> you can always appeal to the Maxwell equations (and to a detailed
> model of how the slit is constructed).  For example, J.D. Jackson
> _Classical Electrodynamics_ analyzes in detail the case of a flat
> plate with a small hole drilled in it, with EM waves leaking out
> through the hole.
>
> OTOH for a wide range of high-school applications, the Huygens
> construction gives an entirely satisfactory qualitative answer.
>
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