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Thermodynamics

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Lippich Polarimeter

Laurent’s polarimeter suffers from the defect that it can be used only for light of a particular wavelength for which the half wave plate has been constructed. To overcome this difficulty, Lippich constructed a polarimeter, that can be used for light of any wavelength.

It consists of two nicol prisms N₁ and N₂. Behind N₁, there is a nicol prism N₂, that covers half the field of view. The nicols N₁ and N₂ have their planes of vibration inclined at a small angle. Suppose the plane of vibration of N₁ is along AB and that of N₂ is along CD. The angle between the two planes is θ. When the analyzer N₂ is rotated such that the plane of vibration of N₂ is along AB, the left half will be more bright as compared to the right half. If the analyzer N₂has its plane of vibration along CD, the right half will be more bright as compared to the left half. YY’ is the bisector of the angle AOC. Therefore, when the plane of vibration of the analyzer N₂ is along YY’, the field of view is equally illuminated. For a slight rotation of the analyzer, either to the right or the left, the field of view appears to be of unequal brightness. Therefore, by rotating N₂, the position for equal brightness of the field of view is obtained. To determine the specific rotation of the optically active substance, the procedure is the same as discussed above.

Three-field system: In the improved form, Lippich polarimeter has a three-field system. The defect in the two-field system is that if the eye is off the axis, even for the position of equal brightness of the field of view, one side appears more bright as compared to the other. Just behind N₁, there are two nicol prisms N₃ and N₄ as shown in fig.

The planes of vibration of N₃ and N₄ are parallel to each other and make a small angle with the plane of vibration to each other and make a small angle with the plane of vibration of N₁. For a particular position of the analyzer N₂, the field of view is equally bright. For any other position of N₂, the field has three parts. The central portion is illuminated by light which has passed through N₁ and N₂, while the other two portions, which are equally bright are illuminated by light passing through N₁. N₂and one of the prisms N₃ and N₄.