  Based on density-functional perturbation theory we have computed from first
principles the photoelastic tensor of few crystalline phases of silica at
normal conditions and high pressure (quartz, $\alpha$-cristobalite,
$\beta$-cristobalite) and of models of amorphous silica (containig up to 162
atoms), obtained by quenching from the melt in combined classical and
Car-Parrinello molecular dynamics simulations. The computational framework has
also been checked on the photoelastic tensor of crystalline silicon and MgO as
prototypes of covalent and ionic systems. The agreement with available
experimental data is good.
  A phenomenological model suitable to describe the photoelastic properties of
different silica polymorphs is devised by fitting on the ab-initio data.
