The theory of electromagnetic waves in a liquid crystal with the dielectric tensor varying from plane to plane is developed, using the method of geometrical optics, or equivalently the WKB approximation. The approximation is shown to apply when the wavelength is slowly varying and when the wavelength is small compared with the pitch of the director. Only normal propagation is considered. Explicit formulae for the propagating fields are derived. It is found that in first approximation the fields are plane polarised in directions that rotate around the normal to the planes in step with the director.