Re: Why called diffraction?

["John S. Denker" <jsd@MONMOUTH.COM>]

On 04/22/2003 08:21 PM, SSHS KPHOX wrote:
> We were talking about X-ray diffraction from crystal lattices in class
> yesterday.

OK.

> The interference that shows the maxima comes from reflection
> from the different planes if I read / understand correctly.

I wouldn't have put it that way.

First of all, there are transmitted diffracted
beams as well as reflected diffracted beams.
So diffraction does not imply reflection.

Secondly, diffracted beams of nonzero order
are tremendously dispersed by wavelength,
unlike ordinary specular reflection.

Thirdly, the "planes" that "cause" the diffraction
are rather abstract and quite numerous, unlike the
single, concrete surface that "causes" ordinary
specular reflection.

Fourthly, you can get one-slit and two-slit
diffraction patterns, which certainly need some
sort of name, so we call them diffraction.

> 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.

When I hear the word diffraction, I think
of outgoing wavelets going off in _all_
directions.  For thin films, there are only
a very few directions involved (incident,
reflected, transmitted).  The directions can
be calculated in the ray-optics approximation
(although you need physical [wave] optics
to get the intensity).  For diffraction, you
need physical optics from the get-go, even
to find the direction (for nonzero order).