An Introduction to Optical Rotation and Polarimetry
In 1808, E.L. Malus discovered that light was not the same in all directions around its line of travel.[3] It showed "two-sideness" or polarization. He observed this when the reflected light was directed onto another reflective glass surface that was kept at the polarizing 57° angle even as it was being rotated about the beam; whereupon, the intensity of the twice-reflected beam was seen to diminish, vanish at one position, and then re-intensify (Fig 1). Evidence of polarization had been seen some years earlier in light that had been passed through doubly refracting crystalline substances like Iceland spar. In 1810, D.F. Arago found that a plate of quartz placed between the reflectors of Malus' apparatus, in the positions where the beam of light vanished, caused the polarized light to rotate; i.e., his second reflector had to be turned to another position for the light to vanish.[4] When J.B. Biot repeated this experiment two years later, he found that the thickness of the quartz affected the extent to which the polarized light rotated. Biot subsequently found other substances capable of causing polarized light to rotate; i.e., solutions of some crystalline solids (like sucrose) and some substances that exist as liquids. Biot attributed the optical activity of these materials to some (then unknown) aspect of their molecular structure. (It is the asymmetry in their molecular structures.) Biot designed one of the first polariscopes (Fig. 2) and formulated the quantitative laws of polarimetry.[5]
Figure 1 - Diagram of apparatus to illustrate the principle of polarization by reflection discovered by Malus in 1808.
Bates, F. J., et al., Polarimetry. Saccharimetry, and the Sugars, Nat. Bur. Stand. (U.S.), Circ. 440, (1942).
Figure 2 - One form of Biot's original apparatus.
Bates, F. J., et al., Polarimetry, Saccharimetry, and the Sugars, Nat. Bur. Stand. (U.S.), Circ. 440, (1942).
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