Liquid Refractive Index Measurement via Geometric Analysis of Total Internal Reflection Light Rings
DOI:
https://doi.org/10.26877/lpt.v5i2.394Keywords:
refractive index, total internal reflection, geometric analysis, physics laboratory experimentsAbstract
Refractive index measurement is a fundamental component of geometric optics; however, determining the refractive index of liquids is often constrained by the high cost of standard equipment and the complexity of direct angular measurement procedures. Consequently, this study proposes an alternative method for determining the liquid refractive index based on the geometric analysis of light ring patterns formed by Total Internal Reflection (TIR). From the ray path analysis, a linear relationship was derived between the square of the light ring radius and the square of the liquid height whereby the refractive index is determined from the gradient of this relationship. The developed method was validated through two experimental schemes determining the refractive index of water and cooking oil and observing the effect of NaCl concentration on the solution’s refractive index. The experimental results demonstrate a linear relationship between and , consistent with the developed equation. The refractive index values obtained for water and cooking oil were 1.339±0.006 and 1.491±0.013, respectively, with relative uncertainties below 1% and accuracies above 98%. In the second experiment, the refractive index of the NaCl solution increased with concentration, although precision limitations in measuring the ring pattern radius caused deviations at several data points. These findings suggest that the reflection light ring phenomenon resulting from total internal reflection can be effectively utilized as an alternative method for measuring liquid refractive indices through a relatively simple experimental approach, with potential for further development in both research and optical education.
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No public repository is currently available for the dataset. However, the instruments and key data summaries used in this study can be obtained from the corresponding author upon reasonable request. Any data shared will be anonymized and provided in accordance with the ethical approval.
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Copyright (c) 2026 Oktaviana Dwi Saputri, Suharto Linuwih, Listiyanto (Author)
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