= Polarimetry
<Polarimetry> measures <optical rotation> with a <polarimeter>. A concrete modern experiment uses the vertical Vernier Go Direct instrument: a $589\,\mathrm{nm}$ light source, fixed <polarizer>, sample cell, motor-driven analyzer, <photodetector> and optical <rotary encoder>. Its readout is transmitted illumination versus analyzer <angle>. Calibrate with the same <solvent> filled to $10\,\mathrm{cm}$, then rinse with sample and fill to $10.0\,\mathrm{cm}$, reading the bottom of the meniscus to $0.1\,\mathrm{cm}$. The shifted maximum nearest zero gives $\alpha$; a fit with the square of a <cosine> uses the whole trace.<Go Direct Polarimeter User Manual by Vernier>[Go Direct Polarimeter User Manual, Using the Product steps 2–4; Specifications; How the Sensor Works]{ref}
In an ideal optical model, <Malus's law> gives $I(\theta)=I_{\max}\cos^2(\theta-\alpha)$ after solvent zeroing. Maxima repeat every $180^\circ$, so tracking the same peak matters. As an illustrative calculation, not a reported measurement, a peak shift from $0^\circ$ to $+5^\circ$ at $l=1.00\,\mathrm{dm}$ and $c=0.100\,\mathrm{g\,mL^{-1}}$ gives $[\alpha]=+50\,\mathrm{deg\,dm^{-1}\,mL\,g^{-1}}$. This calculation says nothing about <R and S configuration>.
The solvent blank controls the baseline. A reference sample and a repeat blank can check scale and drift; record <temperature>, <concentration> and fill height. Use a transparent homogeneous <chemical solution>, and remove bubbles and particles. The manual distinguishes $\pm0.1^\circ$ fit reproducibility from its $\pm1^\circ$ accuracy specification, and warns that a failed fit can give an unreasonable angle. A precise-looking fit therefore need not be accurate.<Go Direct Polarimeter User Manual by Vernier>[Go Direct Polarimeter User Manual, cosine-squared fit in Using the Product; Specifications; Troubleshooting]{ref} Neither a zero reading nor a matching <specific rotation> uniquely identifies a <molecule>; <polarimetry> must be interpreted with the sample composition and these controls.
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