That is, angle through which the plane of the polarized light is rotated on passing through a specific length of solution of known concentration. The experimental arrangement is shown if figure, S is source of monochromatic light placed at the focus of convex lens L. The beam, rendered parallel by lens L, falls on called polarizer P. After passing through polarizer P the light His fundamental striking articles at XAmplified are easy-to-understand and helpful enough for Physics-phobic folks Table Of Content Principle It is used for the measurement of the angle of rotation of optically active substance in solution.
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By measuring the angle e the specific rotation of an optically active substance can be determined. Just after the convex lens there is a Nicol Prism P which acts as a polariser.
H is a half shade device which divides the field of polarised light emerging out of the Nicol P into two halves generally of unequal brightness. 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. Working: 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 by the same angle by which plane of polarization of light is rotated by optically active solution. The position of analyzer is again noted. 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.
To overcome above difficulty half shade device is introduced between polariser P and glass tube T. 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. Determination of Specific Rotation 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.
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 equally dark point is achieved.
The position of the analyzer is again noted. Hence specific rotation S is determined by using the relation.
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 active , and then polarized A and seen through a telescope E. In this basic setup without the half-shade A you are looking for the maximum and minimum brightness, which then tells you that the analyzer A is precisely aligned with the output rotation. The half-shade H goes between the polarized light source and the sample, and it consists of two half-disks of equally absorptive material. One half, ACB, is glass, and it lets the polarized light pass through unchanged. The usefulness of the half-shade is on the sensitivity of the apparatus. This is the core of the method, and it is replayed over and over again in metrology - you get better measurements with differential measurements of quantities that have a sensitive change with respect to the thing you care about, rather than looking at the extrema where the change is more shallow.
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Therefore, the optical rotation depends on temperature, concentration, wavelength, path length, and the substance being analyzed. The polarizer is fixed and the analyzer can be rotated. The prisms may be thought of as slits S1 and S2. The light waves may be considered to correspond to waves in the string.
Laurent’s Half-Shade Polarimeter
By measuring the angle e the specific rotation of an optically active substance can be determined. Just after the convex lens there is a Nicol Prism P which acts as a polariser. H is a half shade device which divides the field of polarised light emerging out of the Nicol P into two halves generally of unequal brightness. 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. Working: In order to understand the need of a half shade device, let us suppose that half shade device is not present.