When polarized light is passed through a substance containing chiral molecules (or nonchiral molecules arranged asymmetrically), the direction of polarization can be changed. This phenomenon is called optical rotation or optical activity.

The light is usually understood to be linearly polarized. The rotation of the orientation of linearly polarized light was observed in the early 1800's before the nature of molecules was understood. Simple polarimeters have been used since this time to measure the concentrations of simple sugars, such as glucose, in solution. In fact, one name for glucose, dextrose, refers to the fact that it causes linearly polarized light to rotate to the right or dexter side. Similarly, levulose, more commonly known as sucrose, causes the plane of polarization to rotate to the left. Invert sugar, formed by converting sucrose to a mixture of glucose and fructose, gets its name from the fact that the conversion causes the direction of rotation to "invert" from left to right.

The degree of rotation depends on the color of the light (the yellow sodium D line near 589 nm wavelength is commonly used), the optical path length, the specific rotation (a characteristic of the material), and the concentration of the material. For a pure substance in solution, if the color and path length are fixed and the specific rotation is known, the degree of rotation can be used to determine the concentration. The polarimeter is a tool of great importance to those who trade in or use sugar syrups in bulk.

The variation in rotation with the wavelength of the light is called ORD . ORD spectra and circular dichroism spectra are related through the Kronig-Kramers transform. Complete knowledge of one spectrum allows the calculation of the other.

In the presence of magnetic fields all molecules have optical activity. A magnetic field aligned in the direction of light propagating through a material will cause the rotation of the plane of linear polarization. This Faraday effect is one of the first discoveries of the relationship between light and electromagnetic effects.

See also: circular dichroism