Centripetal forces modify the optical properties of a rotating glass disk, thus creating a circularly symmetric distortion in the refractive index. This centripetal birefringence has a strong radial dependency and increases with the square of the spin speed. The effect on a polarized beam transmitted through the glass may be reduced mathematically to that of an effective wave plate whose retardance and orientation may be calculated from knowledge of the stress distribution in the disk. Alternatively, one can directly measure the Jones-matrix elements that correspond to the effective wave plate by use of polarization phase measurements at two or more locations on the disk. This direct measurement compensates the centripetal birefringence in the instrumentation employed by the data-storage industry to measure the flying height of read–write heads.