Editing Circular dichroism (section)

=== Optical rotatory dispersion (ORD) ===
CD is closely related to the technique of [[optical rotatory dispersion]] (ORD). Optical rotation is the rotation of linearly polarized light as a result of it passing through an optically active material, and is a consequence of a difference in refractive index between the left and right circularly polarized components of the light (linearly polarized light can be considered to be an equal combination of right and left circularly polarized light). Optical rotatory dispersion is the variation in optical rotation with wavelength. ORD and CD spectra both derive from the interaction of polarized (circularly or linearly) light with chiral molecules and are directly related: each can be derived from the other using the [[Kramers-Kronig transformation|Kramers-Kronig transform]]. Measurements of ORD and CD give essentially the same information, but experimentally each technique has been favored for particular applications. CD is the higher resolution technique since it is only observed at wavelengths where the chiral molecule absorbs light. This makes complicated spectra involving multiple absorbance bands easier to interpret. Alternatively, ORD measurements can be obtained at wavelengths where the chiral molecule being measured does not necessarily absorb light. This means that ORD measurements can be made on materials with high concentrations or where absorbance bands are obscured by solvent or buffer salt absorbance. ORD is particularly useful for unsubstituted sugars as this class of biomolecule does not possess chromophores that absorb in the UV-Vis regions accessible via commercial CD instruments.