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Refractive Index of a Liquid using a Hollow Prism


The refractive index of a liquid can be measured using a hollow prism. For this, liquid is filled inside a hollow prism. To get the refractive index you will use a water prism and get angle of minimum deviation $\delta_m$. The relation between refractive index and minimum deviation is \begin{align} \mu=\frac{\sin\frac{A+\delta_m}{2}}{\sin\frac{A}{2}}. \end{align}

You need a hollow prism, a glass tumbler, chalk piece, laser torch, clamp in which laser can be fixed horizontally, measuring steel tape, sine, and tan tables. This simple experiment can be performed at home.


Glass tumbler is to be used as a stand for placing the prism and the wall is to be used as the screen. Clamp the laser at an appropriate height so that the beam can go through the prism placed on inverted glass tumbler. Make arrangement so that the laser, without the prism, falls perpendicularly on the wall. Prism may be placed about 1.5 m away from the wall.


With the laser switch pressed, and putting the hollow prism on the tumbler, mark the spot on the wall. Put water in the prism so that the laser beam goes through water. The spot will shift. Rotate the tumbler about its axis to get the position of minimum deviation. Measure different distances and calculate the angle of minimum deviation. From this calculate the refractive index of water.

Write all the approximations that you have made and an estimate of errors that could result from the approximations.


  1. Deviation by a Prism
  2. Dispersion of Light
  3. Experiments

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