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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It is a device which is share to measure the optical rotation produced by an optically active substance. By measuring the angle e the specific rotation of an optically active substance can be determined.
It consists of a monochromatic source S which is placed at focal point of a convex lens L. Just after the convex lens there is a Nicol Prism P which acts as a polariser. H is a half shade device lolarimeter divides the field of polarised light emerging out of the Nicol P into two halves generally of unequal brightness.
polatimeter T is a glass tube in which optically active solution is filled. 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.
The rotation of analyser can be measured with the help of a scale C. In order to understand the need of a half shade device, let us suppose that half shade device is not present. The position of the analyzer is so adjusted that the field of view is dark when tube is empty. The position of the analyzer is noted on circular scale. Now the tube is filled with optically active solution and it is set in its proper position.
The optically active solution rotates the plane of polarization of the light emerging out of the polariser P by some angle. So the light is transmitted by analyzer A and the field of view of telescope becomes bright. Now the analyzer is rotated by a finite angle so that the field of view of telescope again become dark. This will happen only when the analyzer is rotated snade the lauremt angle by which plane of polarization of light is rotated by optically active solution.
The position of analyzer is again noted.
optics – Usefulness of half-shade in Laurent half-shade polarimeter – Physics Stack Exchange
The difference of the two readings will give you angle of rotation of plane of polarization 8. 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.
One half ACB is made of glass while other half is made of quartz. Both the halves are cemented together. The quartz is cut parallel to the optic axis. Let us consider that the vibration of polarisation is along OP. The O-component travels faster in quartz and hence an emergence 0-component will be along OD instead of along OC. Now if the Principal plane of the analyzing Nicol is parallel to OP then the light will pass through glass half unobstructed. 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.
When the principal plane of analyzer is along AOB then both halves will be equally bright. On the other hand if the principal plane of analyzer is along DOC. Thus it is clear that if the analyzing Nicol is slightly disturbed from DOC then one half becomes brighter than the other. Hence by using half shade device, one can measure angle of rotation more accurately.
In order to determine specific rotation of an optically active substance say sugar the polarimeter tube T is first filled with pure water and analyzer is adjusted for equal darkness Both the halves should be equally dark point.
Laurent’s Half-Shade Polarimeter
The position of the analyzer is noted with the help of scale. Now the polarimeter tube is filled with sugar solution of known concentration and again the analyser is adjusted in such a way that again poarimeter dark point is achieved.
The position of the analyzer is again noted.
Hence specific rotation S is determined by using luarent relation. It is a polarimeter which can be used to determine specific rotation more accurately.
One half AYB is dextrorotatory while other half is made of laevorotatory quartz as shown in Figure 6. Both the halves are cut with their refracting faces perpendicular to the optic axis. The thickness of each half is approximately 3. One rotates in clockwise while other in anticlockwise.
Red colour is rotated by minimum angle while violet colour by maximum angle. If analyzing Nicol is rotated slightly on either side of. In order to determine specific rotation of an optically active solution say sugar solution whade tube is first filled with pure water and analyzer is adjusted in such a way position that we get tint of passage.
The position of the analyser is noted. Now the polarimeter is filled with sugar solution of known concentration and again the analyzer is adjusted in a position that tint of passage is achieved. Putting this value in equation.
Laurent’s Half Shade Polarimeter | INDOSAW Industrial Products Pvt. Ltd.
Two types of polarimeters are generally used in the laboratory now a days: To overcome above difficulty half shade device is introduced between polariser P and glass tube T.
If the principal plane of analyzing nicol is along AOB then colour of both the halves will be greyish violet coiled tint of passage.
If analyzing Nicol is rotated slightly on either side of this position then one half becomes red and other bluish. Thus we can measure the angle of rotation and hence specific rotation more accurately.