Comprises of two polaroids – fixed polarizer with Laurent’s Half device and rotatable analyzer, mounted at the opposite ends of a tube along the same horizontal. For measurement of optical rotation of plane polarized light by solutions of optically active substances. Comprises of two polaroids – fixed polarizer with. Two types of polarimeters are generally used in the laboratory now a days: (a) Laurent’s Half Shade Polarimeter. (b) Biquartz Polarimeter. Laurent’s Half .

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Many chemicals exhibit a specific rotation as a unique property an intensive property like refractive index or Specific gravity which can be used to distinguish it. Email Required, but polarimeher shown. This tells us how much the plane of polarization is rotated when the ray of light passes through a specific amount of optically active molecules of a sample.

The earliest polarimeters, which date back to the s, required the user to physically rotate polarimeteg polarizing element the analyzer whilst viewing through another static element the detector. Their electric field oscillates in lzurent possible planes relative to their direction of propagation.

It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy. In essence, the polarimeter consists of a collimated linearly polarized light source S, L and P that is passed through the sample tube T which will rotate the polarization if the sample is optically activeand then polarized A and seen through a telescope E.

To overcome above difficulty half-shade device is introduced between polariser P and glass tube T. The introduction of a half-wave plate increased “distinction sensitivity”, whilst a precision glass scale with vernier drum facilitated the final reading to within ca.


A difficulty is faced in the above procedure that when analyzer is rotated for the total darkness, then it is attained gradually and hence it is difficult to find the exact position correctly for which complete darkness is obtained. In other projects Wikimedia Commons. Let us consider that the vibration of polarisation is along OP. According to the direction in which the light is rotated, the enantiomer is referred to as dextro-rotatory or levo-rotatory.

This causes the light to become polarimeyer polarized. Another feature of modern polarimeters is the Faraday modulator. Unsourced material may be challenged and removed. The optical activity of enantiomers is additive. Now the polarimeter tube is filled with sugar solution of known concentration and again the analyser is adjusted in such a way that again equally dark point is achieved.

The position of the analyser is noted. T is a glass tube in which optically active solution is filled.

Therefore, the optical rotation depends on temperature, concentration, wavelength, path length, and the substance being analyzed. In addition, they provide continuous measurement, facilitating High-performance liquid chromatography and other kinetic investigations.

Hence glass half will be brighter than quartz half or we can say that glass half will be bright and the quartz half will be dark. That plane is called the plane of polarization. The position of the analyzer is noted on circular scale. In order to understand the need of a half shade device, let us suppose that half shade device is not present.


If analyzing Nicol is rotated slightly on either side of. More importantly, though, polarimefer comparing two signals that have positive slope, and this does wonders for the sensitivity. Now the analyzer is rotated by a finite angle so that the field of view of telescope again become dark. Advanced Organic Chemistry, Part A: The normal polarimeter requires the experimenter to look for the maximum or minimum brightness, and this is a tricky thing to do: Sign up or log in Sign up using Google.


The half-shade H goes between the polarized light source and the sample, and it consists of two half-disks of equally ehade material.

Polarimeter | Physics Assignment

It consists of a monochromatic source S which is placed at focal point of a convex lens L. You can not accurately judge the position of maximum brightness without using half-shade plate, which may lead you an inaccurate measurement of the optical activity of the experimental liquids.

Without using half shade device your eye cannot judge the exact position of extinction of light when the two nicols are placed in polarimetee crossed position. The difference of the two readings will give you angle of rotation of plane of polarization 8. Putting this value in equation. Webarchive template wayback links Articles needing additional references from December All articles needing additional references Commons category link is on Wikidata.

Laurent’s Half-Shade Polarimeter

The light after passing through T is allowed to fall on the analyzing Nicol A which can be rotated about the axis of the tube. Shaona Bose 2 6 The optically active solution rotates the plane of polarization of the light emerging out of the polariser P by some angle.

It is a polarimeter which can be used to determine specific rotation more accurately. If light encounters a polarizer, only the part hal the light that oscillates in the defined plane of the polarizer may pass through.

The polarizer is fixed and the analyzer can be rotated. Both the halves are cemented together. Often, the sugar refineries use a modified polarimeter with a flow cell and used in conjunction with a refractometer called a saccharimeter.