The refractive index, polarizability and thermodynamic response of polystyrene under shock compression were investigated through experiments and theoretical analysis, and a relationship between the refractive index and the density, pressure and temperature of the polystyrene was obtained. Above a pressure of 20 GPa, an obvious inflexion was observed in how the refractive index of the polystyrene varied with the thermodynamic variables; in particular, it was found to depend strongly on the temperature (as obtained using a semiempirical complete equation of state). Relating the measured refractive index to the polarizability indicates that the polarizability decreased from 1.28 x 10(-23) cm(3) at ambient conditions, to 0.98 x 10(-23) cm(3) at pressure of 33 GPa, indicating that the lowest direct band gap of polystyrene becomes < 2 eV, similar to that of many semiconductor materials.