ABSTRACT
Optical rotation refers to the effect on a plane polarized beam of light as it passes through a chiral medium. A plane polarized beam of light can be thought of as the superposition of two circularly polarized beams, of equal amplitude and opposite senses of polarization. The most commonly observed phenomenon is a reorientation of the plane of polarization, ‘‘circular birefringence.’’ Thismaybe considered a consequence of adifferent indexof refraction for the left-and right-polarized components, nL and nR. Moscowitz [1] writes
a ¼ p l (nL nR)
where a is the optical rotation due to circular birefringence in radians per centimeter and l is the wavelength of light used in the measurement. The optical rotation is dependent on wavelength and temperature; the specific rotation is defined at a given temperature T (usually 258C) and wavelength l (usually the Na D-line at 589 nm) as
[a]Tl ¼ a
c : 1800
p
Here c is the concentration in grams per milliliter of solution and 1800=p is the conversion giving specific rotation in degrees per decimeter. The sample cells are usually cylindrical tubes 1 or 2 decimeters in length, long enough to allow observably large rotation. The rotation also depends on the concentration of the chiral material. This factor appears in the definition of the molecular rotation
[F]Tl ¼ [a]Tl:M 100
where M is the molecular weight of the optically active compound.
