POLARIMETRY - Discussion

Discussion:
Source of monochromatic light is placed at the focus of convex lens. The beam, rendered parallel by lens, falls on polarizer. After passing through polarizer the light becomes plane polarized. The polarized light beam passes through a half-shade device and then through a tube containing optically active solution. The transmitted light passes through another Nicol which can be rotated about the direction of propagation of light as axis and its rotation can be read on a circular scale graduated in degree, with the help of a Vernier scale. 
This experiment is used for the measurement of the angle of rotation of optically active substance in solution. That is, angle through which the plane of the polarized light is rotated on passing through a specific length of solution of known concentration. 
In the experiment, sugar is dissolved completely in water so that concentration of solution is exact. Air bubbles are removed to reduce errors. Temperature during experiment is recorded because optical activity depends on temperature. Failure to judge the equality of brightness will introduce errors. To minimize those errors, four readings should be taken for each concentration at intervals of ≈180 degrees. 
Angle of rotation of water is zero. Now, the angle of rotation of sugar solution is the angle at which intensities of both (right and left) sides are equal and bright. This is by half shade principle.