yyElectrooptic properties of colloidal crystals of silica spheres (diameter: 103 nm) in an exhaustively deionized aqueous suspension have been studied by electric reflection spectroscopy using the rectangular parallelepiped-shaped cells with electrical membranes inside the cell wall. Acoustic shear waves are induced inside and even outside the electrodes, where the electric field is absent, when sine-wave electric fields ranging 0.01-1000 Hz in frequency and 0.0001-100 V/cm in field strength are applied. Modulation effects such as phase delay, change in the amplitude, and harmonics generation are observed. A significant effect of the optical path length is observed on the shear waves; i.e., the amplitude and phase difference change significantly for the cell of long optical length. The importance of the synchronous fluctuation of the colloidal spheres mediated by the expanded electrical double layers in the crystal lattice is strongly supported for the electrooptic effects observed.