Optical rotation, also known as optical activity, is the phenomenon where the plane of polarized light is rotated when it passes through certain substances. This effect is observed when light interacts with chiral molecules — that is, molecules that cannot be superimposed on their mirror images, similar to how left and right hands are different.
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Optical rotation is the signed angle measured in polarimetry. Positive rotation is clockwise for an observer facing the incoming light; negative rotation is counterclockwise. State the viewing convention, wavelength, temperature, solvent, concentration and path length when comparing measurements. R and S configuration specifies spatial arrangement, and does not predict the sign of optical rotation.[ref]
5.5 Sequence Rules for Specifying Configuration. McMurry, Organic Chemistry, §5.5, discussion after Figure 5.10: no simple relation between R/S and rotation sign
Here the observer stands beyond the sample, on the detector side, looking back toward the source. Measure both the polarimeter analyzer angle and the optical rotation clockwise from the blank's polarization direction. Looking from the source end reverses the apparent sense. A line has no distinguished arrowhead, so an intensity scan alone determines its orientation only modulo ; continuity or additional information must choose the appropriate rotation branch.