To realize a high-precision refractive-index measurement by the rainbow method by use of multifringes, the characteristics of several orders of supernumerary bow fringes are made clear by a simulation based on the Huygens-Fresncl principle. The fringe intervals computed are precisely coincident with those obtained in experiments. As a result a suitable combination of the diameter of the cylinder containing the sample and the beam size of thc laser has been dctermined. By usc of the characteristic curve of the deviation angle versus the refractive index for each fringe and by thc statistical treatment, measurement with high precision was demonstrated.
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