Measurement of permittivity using an open resonator

When a flat sheet of dielectric is in an open resonator, its surfaces do not coincide with the curved wavefronts of the electric field. For calculations, the specimen may be imagined to be composed of a specimen having curved faces plus a thin concave lens, whose refractive index must be supposed to vary with the position of the lens in the field. The results of measurements, using specimens of polystyrene and of alumina, show that a satisfactory theory can be developed on this basis, leading to values of refractive index n agreeing with those of the earlier theories. The new theory, however, predicts slightly different beam widths, which have been confirmed by measurement.