A system such as a vehicle may have window that exhibit adjustable transparency. The windows may include liquid crystal devices and polymer dispersed liquid crystal devices that exhibit adjustable amounts of light transmission and haze. An optical property of a window such as window transparency may be modulated using an alternating-current modulation waveform. Modulation of the transparency of the window may be synchronized with modulated light output from a light source. The light source may be located inside the vehicle or may be located outside of the vehicle. By synchronizing the modulation of the transparency of the window with the light source output, privacy may be enhanced or glare may be reduced.