Chiral nematic liquid crystals which possess a large negative dielectric anisotropy (Delta epsilon < 0), exhibit a larger dielectric constant parallel to the helical axis than perpendicular to it. In a liquid crystal eel with homeotropic boundary conditions, the direction of the helical axis is parallel to the cell surfaces and the sample adopts a pseudo focal conic texture. On the application of a suitable AC field, the director is rotated, such that the helix direction is now perpendicular to the surface and the sample adopts a Grandjean texture. The wavelength of reflected light in this "field On" state is a function of temperature and the magnitude of the applied field. In this paper we show that highly twisted chiral nematic liquid crystals, which possess a large negative dielectric anisotropy are potentially useful as optical modulators and in electrically tuneable mirrors.