LAURENT HALF SHADE POLARIMETER PDF
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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So, is the presence of the half-shade benefiting the experiment? The principles of the Laurent half-shade polarizer are well explained in this tutorial.
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. 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. In this version of the polarimeter, you’re looking for the point where both halves are exactly equal in brightness, i.
The usefulness of the half-shade is on the sensitivity of the apparatus. The normal polarimeter requires the experimenter to look for the maximum or minimum brightness, and this is a tricky thing to do: With the half-shade, on the other hand, you’re comparing two different signals at the point where they match, which means that you have an immediate reference the signals are equal or they’re not, shzde you don’t have to fiddle with the analyzer angle to check whether you’re at the correct spot or not.
More importantly, though, you’re comparing two signals that have positive slope, and this does wonders for the sensitivity.
It is immediately obvious that the larger slope makes the ‘shadow’ on the angle axis, which gives the range of uncertainty is much smaller. It’s important to emphasize that this is not just a one-off sort of difference: 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.
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.
The half-shade plate leads you to accurate measurement of optical activity by helping you to determine the exact position of the equal brightness of the two halves.
Without using half shade device your eye cannot judge the exact position of extinction of light when the two nicols are placed in the crossed position.
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