The production of an elliptically polarized wave when a linearly polarized light wave passes through a substance that has differences in the extinction coefficients for left- and right-handed polarized light. The size of this effect is given by φ‎ = π‎/λ‎(η‎_l – η‎_r), where φ‎ is the ellipticity of the beam that emerges (in radians), λ‎ is the wavelength of the light, and η‎_l and η‎_r are the absorption indices of the left- and right-handed circularly polarized light, respectively. Circular dichroism is a property of optically active molecules and is used to obtain information about proteins. See also CD spectrum.