Spin observables and nuclear geometry

The new measurements of polarization $P$ and spin rotation function $Q$ in 500 MeV $p$-${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ elastic scattering require geometric differences among the various parts of the $p$-nucleus interaction. We present a general analytic formalism which defines these differences and shows how the new experiments can be interpreted as interferometric determinations of them.