  The Kapitza - Dirac effect is the diffraction of a well - collimated particle
beam by a standing wave of light. Why is this interesting? Comparing this
situation to the introductory physics textbook example of diffraction of a
laser beam by a grating, the particle beam plays the role of the incoming wave
and the standing light wave the role of the material grating, highlighting
particle - wave duality. Apart from representing such a beautiful example of
particle - wave duality, the diffracted particle beams are coherent. This
allows the construction of matter interferometers and explains why the Kapitza
- Dirac effect is one of the workhorses in the field of atom optics. Atom
optics concerns the manipulation of atomic waves in ways analogous to the
manipulation of light waves with optical elements. The excitement and activity
in this new field of physics stems for a part from the realisation that the
shorter de Broglie wavelengths of matter waves allow ultimate sensitivities for
diffractive and interferometric experiments that in principle would far exceed
their optical analogues. Not only is the Kapitza - Dirac effect an important
enabling tool for this field of physics, but diffraction peaks have never been
observed for electrons, for which is was originally proposed in 1933. Why has
this not been observed? What is the relation between the interaction of laser
light with electrons and the interaction of laser light with atoms, or in other
words what is the relation between the ponderomotive potential and the
lightshift potential? Would it be possible to build interferometers using the
Kapitza - Dirac effect for other particles? These questions will be addressed
in this paper.
