Optical rotatory dispersion (ORD) measures the change in optical rotation of a substance with changing wavelength of light. ORD can be used to determine the absolute configuration of chiral molecules like metal complexes. When plane-polarized light passes through a chiral substance, the left and right circularly polarized components propagate at different speeds, leading to optical rotation. Measuring optical rotation as a function of wavelength is ORD spectroscopy. The Cotton effect occurs near absorption bands, where peaks and troughs in the ORD curve depend on whether absorption first increases or decreases with wavelength. Applications of ORD and circular dichroism include determining structures of amino acids, proteins, steroids and antibiotics.