High precision Refractive-index Measurement Using Rainbow Method and Its Application to Water Analysis

一种简单、高精度的测量液体折射率的方法不仅适用于材料分析，也适用于河水和海水的环境污染监测。我们开发了一种测量液体折射率的新方法，该方法利用了包含液体样品的圆柱形电池中激光束的最小偏差。其原理与自然彩虹的生成几乎相同，因此我们称这种方法为“彩虹法”。将待测液体装在直径为60mm的圆柱形玻璃池中，将氦氖激光射入池中，使激光入射满足最小偏差条件。在这种情况下，从单元中出现的光束具有干涉条纹，该干涉条纹可用作标记来测量光束的微弱偏转。基于惠更斯-菲涅耳原理对光学系统进行了计算机模拟。计算得到的条纹形状和条纹间距与实验结果吻合较好。结果表明，通过计算机模拟研究，可以确定样品柱直径与激光光束尺寸的合适组合。利用各条纹偏差角与折射率的特征曲线，并进行统计处理，证明了测量精度高。在水质分析中也进行了一些应用，取得了较好的效果。

A simple and high precision method to measure the refractivity of a liquid has been required for material analysis and also for application to environmental pollution monitoring of river water and sea water. We have developed a new method for measuring the refractive indices of liquids, This method uses the minimum deviation of a laser beam in a cylindrical cell that contains the liquid sample. The principle is nearly the same as that of the generation of a natural rainbow, and we therefore call this method the "Rainbow Method". The liquid to be measured is contained in 60 mm diameter, cylindrical glass cell, and a He-Ne laser light is passed into the cell so that the laser light incidence fulfils the condition of minimum deviation. In this condition, the beam emerging from the cell has a interference fringe which can be used as a marker to measure the faint deflection of the laser beam. Based on Huygense-Fresnel principle a computer simulation was made on the optical system. The fringe shape and the fringe intervals computed are precisely coincident with those obtained in experiments. As a result, suitable combination on the diameter of the cylinder containing a sample and the beam size of a laser can be determined by the computer simulation study. By using the characteristic curve of deviation angle vs. refractive index for each fringe, and by the statistical treatment, measurement with high precision was demonstrated. Some application to the water analysis also has been carried out with fairly good result.