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| Determination of Optical Activity using a Polarimeter |
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| Two Nicol prisms (got by combining two prisms of Iceland spar suitably cut) are placed between the source of monochromatic light, S, and the eye. Light passes through the first Nicol prism, P, the polarizer and gets plane polarized (vibrations are restricted to one plane only). |
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| This plane polarized light then passes through the second Nicol Prism, A, the analyzer and reaches the eye. However when the axis of the analyzer is perpendicular to that of the polarizer, no light reaches the eye. |
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| fig.12.9 - Plane of polarization of Plane Polarized Light is rotated by cane-sugar solution |
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| The polarizer and analyzer are arranged in such a way that no light reaches the eye. A tube T filled with cane sugar solution is placed between the two prisms. Some light can be seen. The analyzer should be rotated so that once again no light reaches the eye. |
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| Note that some light could be seen when the cane-sugar solution tube was placed between the two prisms whose axes were perpendicular to each other and no light was visible originally. So what made this light visible on the right side when neither of the two prisms had been moved? Light is visible only if the plane of polarization has been rotated. The answer is cane sugar solution has rotated the plane of
polarization. |
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| Substances like cane sugar that are capable of rotating the plane polarized light are optically active and the phenomenon is called Optical Activity. Substances that rotate the plane of polarized light to the right are called dextro rotatory and those that rotate it to the left are called laevo rotatory. |
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| Polarimeter is the instrument used to measure the angle of rotation of a plane polarized light. The magnitude of rotation depends on: |
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| i) Nature of the compound |
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| ii) Concentration of the solution |
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| iii) Thickness of the layer traversed by polarized light |
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| iv) Nature of the solvent |
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| v) Wavelength of the light used |
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| vi) Temperature of the solution. |
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