Polarimeter is a reliable instrument for accurate measurement of Optical Rotations, for study of Polarization effects and for quick and easy discrimination between right-handed sugars such as Dextrose and left handed sugar such as levulose. The instruments find use in industries as well as in Colleges and Universities.
The Polarimeter essentially consists of a pair of polarizes arrange in a straight line, one of which (analyzer) can be rotated with respect to the other (Polaris) and the rotation measured on a divided circle. The analyzer is rotated till it is ‘crossed' with the Polaris. In this position the planes of he polarization of the polarizer and the analyzer, are mutually perpendicular, so that there is no transmission of light across the pair. Now if a tube containing an optically active solution is introduced between the polarizer and the analyzer, the light transmission is partially or wholly (if the optical rotation of the liquid equals90 o ) resorted. The analyzer is rotation again to obtain the “no transmission” position. This rotation equals, the optical rotation of the liquid.
Since it is difficult to recognize the actual “no transmission” position, one is able to obtain a ”range” only for no transmission. The definition of this range is quite subjective. To overcome this difficulty, in modern polarimeter, an extra device is used. This device is a “half-wave plate”, covering half of the circular field of view. This plate introduces a certain path difference between light waves in the two halves of the field of view. Thus when the analyzer is rotated the intensity in one half of the field of view is extinguished slightly before the other half. At this point it is easy to continue rotation of the analyzer to obtain a uniform illumination of the field of view. This position can be determined quite accurately and is used as the desired position. The optically active substance (solution) is now introduced between the polarize and the analyzer and the uniform, field of view position is determined again to obtain the optical rotation of the liquid.
The circular scales are finely marked on a 10cm diameter circle both in angular measure and in International Sugar Scale. The smallest division on angular scale is 1 o ISS. The Sugar Scale is calibrated between—30 o (for Laboratory substances) and +130 o ISS. A divided of 0.05o(3 minutes) on the angular scale and 0.1 o ISS. The Scale is properly illuminated to facilitate reading.