Plane-Polarized Light and Handedness

It is important to understand that plane-polarized light can be produced both naturally (calcite and quartz crystals) and synthetically (with aligned polymers [Chapter 6]), and when such polarized light passes through chiral ( handed ) materials (or solutions of chiral materials), the plane of polarization is rotated. Historically, this property has been called optical rotation, and the chiral material was said to be optically active. Indeed, experimentally, the rotation of the plane of plane-polarized light demonstrates optical activity and thus, that the material is chiral. 

Subsequently, in 1828, Nicol (W. Nicol 1770-1851, Edinburgh) constructed an optical device (the Nicol prism) made from calcite, cut so as to remove one of the two refracted rays by total reflection. Therefore, only passage of a single beam of plane-polarized light (i.e., light confined to one plane) is allowed through a Nicol prism. 

Because a chiral object and its realized mirror image interact oppositely with the circularly polarized light (a perturbation in the bonding electrons if not an absorption), in the presence of one of the enantiomers the velocity of one of the circularly polarized components of the light will be effected more than the other so that, on recombination as the beam leaves the material, it will be seen to have had its direction of oscillation, and hence its plane of polarization, rotated relative to its original position. 

The extent of such clockwise or anticlockwise polarization is measured in a device called a polarimeter, which consists of two Nicol prisms cemented together with a Canada balsam glue. The first, called the polarizer, is used to polarize the light source. The second, called the analyzer, is used to measure the extent to which the beam of plane-polarized light has been rotated by the sample. A polarimeter is schematically shown as Rgure 4.61. 

X = wavelength oftheUghtused(frequentlytheDline of sodium[589.3nm]) a = observed rotation  

---