An interferometric method is applied to study piezoelectricity in quartz and electrostriction in SiO2-glass. With the application of an AC-potential at 50 Hz on Au-electrodes, the reflection intensity of polarized He-Ne laser light from partially transparent Au/quartz (AT-cut)/Au interfaces is found to oscillate at 50 Hz. The oscillating amplitude is proportional to the amplitude of the applied sinusoidal electric field. The reflection intensity from partially transparent Au/SiO2-glass/Au interfaces oscillates at twice the frequency of the applied AC-potential, i.e. at 100 Hz, with the oscillating amplitude proportional to the square of the amplitude of the applied sinusoidal electric field. The relationships between reflection intensity and applied electric field strength are derived for piezoelectricity and electrostriction, respectively.