Measuring the refractive index (RI) of a liquid without using any fancy equipment has become a popular topic in various STEM classses and events.1–6 This article proposes a simple method that has the following properties:

Our method is motivated by Gluck and Massalha,2 who used a trigonometric approximation to reduce the number of unknowns in deriving their RI formula. Unfortunately, the approximation means the loss of property (i). In fact, there seems to exist no RI experiment with household items that possess all the aforementioned virtues.

Our experiment uses a “bridge”-shaped paper slip with fixed widths; see Fig. 1(a). Given a fish tank or aquarium with thin transparent walls, and some liquid inside, glue the bridge-shaped paper slip on one side of the fish tank, and a white sheet of paper on the opposite side. (Stick the paper on the outside of the fish tank to avoid...

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