It consists of two semis circular plates.
One is of quartz and other is glass plate joined together along the diameter. The quartz semi circular plate is a half wave plate whose optic axis is parallel to common diameter. It is made by cutting quartz plate whose faces are parallel to the optic axis and has thickness (n + ½) l, l being wavelength of light used. The other semi circular plates of glass have thickness such that both the plates transmit light of equal intensity. The lines in left half show direction of optic axis and it is a HWP of quartz
Working: Light from monochromatic source is incident on converging lens. After passing through polarizer (Nicol prism) A1 the beam is plane polarized. One half of the beam passes through glass plate and the other half through quartz plate Q (as showing in fig. 2.4).
Suppose the plane vibration of PPL incident of half wave plate is along AB. Here AB makes an angle ? with the direction of incident PPL. On passing through angle ?, the beam is split into ordinary and extra ordinary beam. A path difference of is introduced between them. The vibration of the beam emerging out of the quartz is along CD and those emerging out of glass are along AB.
On passing through the analyzer if A1OA2 has its principal section along AB the amplitude of light incident on the analyzer from both the halves ( Q or G ) will be equal and the view will be equally bright.
If the analyzer plane A1OA2 is rotated to the right of AB is the right half will be brighter than left half. Similarly, if we rotate N2 to the left, the left half will be much brighter. Therefore, to find the specific rotation of an optically active substance the analyzer is set for equal brightness with the water in polar meter tube and note down the reading. Now if we remove the water and filled the tube with sugar solution the vibration from quartz and glass plate are rotated clockwise. So the field is not equally bright. The analyzer is rotated clockwise for equal brightness and then takes reading. The difference between both the readings gives the optical rotation