Re: Why called diffraction?

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

HI,
        I have wondered this myself.  I think Ken's
question is 'Why the word "diffraction" rather
than "interference?" '  If a crystal consist of
only two planes, the analogy with thin film
interference would be very close except for the
crystal have a discrete structure.  Thin films
exhibit dispersion the same as crystals.

        My first guess is it is historical with the
patterns seem experimentally first.  These
patterns appear to be some kind of a
generalization of the pattern from  diffraction
gratings.  Thus the pattern seen by directing x
rays through crystals was called diffraction.

        In my mind, the concepts of interference and
diffraction are deeply intertwined.  Is it a
single slit interference pattern or a diffraction
pattern?  In a diffraction pattern, the bright
spots occur where there is constructive
interference.  When light diffracts around a
corner, one figures out what is happening by
calculating the interference.  Many  interference
patterns depend on light being diffracted around
an edge or object.

        I cannot think of a case of diffraction without
interference.  But there is interference without
diffraction.  Examples include a water ripple
thank with two oscillating objects in the water;
twin radio antennae, and maybe thin film
interference.

Thanks
Roger Haar







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Brian Whatcott wrote:
> At 06:21 PM 4/22/2003 -0600, you wrote:
> > We were talking about X-ray diffraction from crystal lattices in class
> > yesterday. The interference that shows the maxima comes from reflection
> > from the different planes if I read / understand correctly. So one of my
> > bright students asks "why" is it called diffraction? Thin film
> > interference is not called diffraction and yet it is interference due to
> > reflections.
> >
> > I had no answer. Why is it called diffraction? Or is my understanding of
> > the process flawed?
> >
> > Thanks
> >
> > Ken Fox
>
> I'll take a shot at this one:
>
> the central idea of diffraction is the bending of a beam through an
> aperture, or
> in passing a line feature, with selective reinforcement and cancelation of
> the beam.
>
> Refraction is the change in direction of a wavefront  passing obliquely through
> a medium of graded or discontinuous propagation speed.
>
> Reflection is the abrupt change in direction of a wavefront at a well defined
> smooth surface between materials of  discontinuous propagation speeds.
>
> Considering x ray diffraction, the wavelength is similar to the lattice spacing
> and the beam is bent with contributions from multiple surfaces in the crystal.
>
> Brian Whatcott    Altus OK