Using minimum deviation of a secondary rainbow and its application to water analysis in a high-precision, refractive-index comparator for liquids

A new method for measuring the refractive-index difference of a liquid has been developed. The liquid to be measured is contained in a 60-mm-diameter, cylindrical glass cell, and a He-Ne laser light is passed into the cell so that the laser light incidence fulfills the condition of minimum deviation. In this condition, the beam emerging from the cell has a fine interference fringe. The position of the interference fringe is read out as a marker to measure the deflection of the laser light. Directly reading the peak shift of the interference fringe makes it easy to obtain the refractive index difference of the liquid with a fairly high precision of at least 6 x 10(-6). Further high precision is potentially expected to be realized by use of an improved data analysis treatment of the overall interference fringe pattern.