  A fraction of the light observed from edge-on disk galaxies is polarized due
to two physical effects: selective extinction by dust grains aligned with the
magnetic field, and scattering of the anisotropic starlight field. Since the
reflection and transmission coefficients of the reflecting and refracting
surfaces in an optical system depend on the polarization of incoming rays, this
optical polarization produces both (a) a selection bias in favor of galaxies
with specific orientations and (b) a polarization-dependent PSF. In this work
we build toy models to obtain for the first time an estimate for the impact of
polarization on PSF shapes and the impact of the selection bias due to the
polarization effect on the measurement of the ellipticity used in shear
measurements. In particular, we are interested in determining if this effect
will be significant for WFIRST. We show that the systematic uncertainties in
the ellipticity components are $8\times 10^{-5}$ and $1.1 \times 10^{-4}$ due
to the selection bias and PSF errors respectively. Compared to the overall
requirements on knowledge of the WFIRST PSF ellipticity ($4.7\times 10^{-4}$
per component), both of these systematic uncertainties are sufficiently close
to the WFIRST tolerance level that more detailed studies of the polarization
effects or more stringent requirements on polarization-sensitive
instrumentation for WFIRST are required.
