This study describes the observation of large induced refractive index changes produced by $157\mathrm{nm}$ excimer laser exposure in high-purity synthetic silica glasses. With $157\mathrm{nm}$ exposure, large induced changes are observed within a few hundred thousand pulses of exposure. Similar to $193\mathrm{nm}$ exposures, exposure with polarized $157\mathrm{nm}$ light yields polarization-induced birefringence (PIB). However, the $157\mathrm{nm}$ exposure also exhibits a behavior not observed with $193\mathrm{nm}$ exposures; namely, the initial response of the glass is a decrease in refractive index, followed by an increase with continued exposure. An explanation of the behaviors for both wavelength results is proposed where the induced refractive index is considered to arise from two different concurrent phenomena. One produces a decreased refractive index and also accounts for the PIB. The other, which accounts for the increased refractive index, is associated with an isotropic laser-induced volume change.