Optical rotation occurs because optically active samples have different refractive indices for left- and right-circularly polarized light. Another way to make this statement is that left- and right-circularly polarized light travel through an optically active sample at different velocities. This condition occurs because a chiral center has a specific geometric arrangement of four different substituents, each of which has a different electronic polarizability. Light travels through matter by interacting with the electron clouds that are present. Left-circularly polarized light therefore interacts with an anisotropic medium differently than does right-circularly polarized light.
Linearly or plane-polarized light is the superposition of equal intensities
of left- and right-circularly polarized light. As plane-polarized light
travels through an optically active sample, the left- and right-circularly
polarized components travel at different velocities. This difference in
velocities creates a phase shift between the two circularly polarized components
when they exit the sample. Summing the two components still produces linearly
polarized light, but at a different orientation from the light entering
the sample.
Schematic of a polarimeter
When an optically active substance is present in the beam, it rotates the polarization of the light reaching the analyzer so that there is a component that reaches the detector. The angle that the analyzer must be rotated to return to the minimum detector signal is the optical rotation, α.
The amount of optical rotation depends on the number of optically active species through which the light passes, and thus depends on both the sample path length and the analyte concentration. Specific rotation, [α], provides a normalize quantity to correct for this dependence, and is defined as:
[α] = α/l·d
The optical rotatory dispersion (ORD) is the optical rotation as a function
of wavelength. It is recorded using a spectropolarimeter, which has a tungsten
lamp and a scanning monochromator as the light source. A motorized mount
rotates the analyzer to maintain a minimum signal at the detector. Usually
a modulation is introduced into the polarization angle of the light beam,
so that a DC signal to the analyzer motor then keeps the detector signal
centered at the minimum value.