The liquid-crystal molecules can initially be aligned parallel to the cell walls with a transverse alternating field (Fig. la). The device then behaves as a uniaxial crystal having its optic axis parallel to the cell wall and a normal polarised beam will pass through undeflected. By varying the ratio of the electrode potentials a bent or 'clinotropic' alignment is induced, which defines a new spatially varying optic axis (Fig. ). For light polarised in the plane of this new optic axis, the optical path length through the cell will vary from one end to the other, and radiation will be refracted. In some respects the device behaves as a lens or prism, except that it is polarisation dependent and light follows a curved path through the cell.