  The momentum changes caused by position measurements are a central feature of
wave-particle duality. Here we investigate two cases - localization by a single
slit, and which-way detection in the double-slit interference experiment - and
examine in detail the associated momentum changes. Particular attention is
given to the transfer of momentum between particle and detector, and the recoil
of the measuring device. We find that single-slit diffraction relies on a form
of `interaction-free' scattering, and that an ideal which-way measurement can
be made without any back-reaction on the detector.
