SM205     Nicol’s prism and Polarimeter      circa 1950     Light

  Nicol’s prism is one of the most valuable means of producing a polarised beam of light since it polarises it completely using the property of Iceland spar of a differing refractive index in different axes of the crystal.   The prism is constructed from a Rhombohedron of Iceland spar about 2.5 cm high and 8 mm wide.   It is bisected through a plane passing through the obtuse angles, and the two halves are again joined by means of Canada balsam.   The Canada balsam has a refractive index of 1.549, while the ordinary index of Iceland spar being 1.654 and the extraordinary index being 1.483.   The special construction of the prism will totally polarise light passing through it and two of these prisms are used in the polarimeter that can measure the rotation of the polarisation plane occurring in solutions of certain organic compounds.

 In the polarimeter (SM205), the solution is placed in a glass tube with optically flat ends.   Monochromatic polarised light enters one face of the tube and the light passing through the solution arrives at the eyepiece after passing through a second Nicol prism at right angles to the first.   Since the solution will introduce rotation of the plane of polarisation proportional to its concentration, the second Nicol prism will have to be rotated on a graduated scale to bring it in a position of minimum light passing.   The angle turned by the second Nicol prism will be the rotation (clockwise or anticlockwise) due to the solution.

The rotation, a is related to the quantity of active substance such that:  

a = k l x

  where l  is the length of the tube

            x  is the concentration

 and     k  is a constant called the specific rotation of the substance.