The science of measuring the change in polarization orientation at a single wavelength is called polarimetry, and the measuring instrument is called a polarimeter. The measurement of the change in polarization angle over a range of wavelengths is known as optical rotatory dispersion (ORD). These measurements are useful for studying the structure of anisotropic (directionally dependent) materials and for checking the purity of chiral mixtures. A sample containing an equimolar mixture of enantiomers is called a racemate and a sample that contains only one enantiomer of a chiral molecule is said to be optically pure. The enantiomeric ratio, abbreviated as ee%, is defined as the percentage of one enantiomer in a mixture to that of the opposite enantiomer. The enantiomer that rotates light to the left, or counterclockwise when viewing in the direction of light propagation, is called the levorotatory or (−) enantiomer, and the enantiomer that rotates light to the right, or clockwise, is called the dextrorotatory or (+) enantiomer. Other enatiomer nomenclatures such as D, L and R, S are structural descriptions. As a result they are not necessarily indicative of absolute optical activity.

The phenomenon of optical rotation occurs because optically active samples have different refractive indices for left- and right-circularly polarized light. Restated, the 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. Interactions occur between the light traveling through the medium and 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 can be represented as 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. The summation of the two components still produces linearly polarized light but at a different orientation from the light entering the sample. This change in orientation, expressed as an angle deviation from the input polarization state, is known as optical rotation. When an absorption band is in electronic contact with the chiral center, a differential absorption of the circularly polarized light components occurs. Now when the two components are summed, elliptically polarized light is produced, and the major axis is rotated compared to the starting orientation. This phenomenon is known as circular dichroism (CD).

Magnetically induced optical rotation occurs in many solids, liquids, and gases. This rotation, called the Faraday effect or Magneto-Optic Effect was discovered by Michael Faraday in 1845, was the first experimental evidence that light and magnetism are related. This effect occurs in most optically transparent dielectric materials when they are subject to strong magnetic fields. The magnitude of the optical rotation depends upon the strength of the magnetic field, the temperature, the frequency of the light, the observed path length, and Verdet's constant, which is an intrinsic property of the transmitting substance. By convention, a positive Verdet constant is the material property corresponding to a L-rotatory (anticlockwise) Faraday rotation when the direction of propagation is parallel to the magnetic field and a R-rotatory (clockwise) Faraday rotation when the direction of propagation is anti-parallel to the magnetic field. Therefore the rotation direction is the same even when the light propagation direction is reversed. So if a ray of linearly polarized light is passed through a material along the axis of an applied magnetic field and reflected back along the same path, the rotation doubles. In contrast, if a ray of linearly polarized light is passed through a material with natural optical activity and reflected back along the same path, the rotations cancel and the net rotation is zero.