When placed between two plates with alternating electrical polarity these molecules tend to align themselves with the field. They alter their alignment to keep in step with the changing polarity of the plates. This is the description of a device known as a condenser. Alignment is favored when the temperature is low. This value approaches the square of the index of refraction of the substance.
Many references discuss the principles involved.
A few references are listed at the end of this write up. A drawing and photographs of the cell can be viewed below. A drying column was attached to the cell to prevent contamination of the test media by water condensing from the atmosphere when test temperatures were less than the dew point. To reach low temperatures a mixture of methanol and liquid air was used in the beaker. Some of this data is plotted in the graph below.