Many amino acids exist as chiral molecules
Chiral molecules are mirror images or enantiomers. There is no symmetry operation in 3D-space that can be performed on one enantiomer to make it overlay the other. Enantiomerism is a special form of isomerism. The physical properties of enantiomers are identical in every way except two: how the molecules interact with polarized light and how they interact with other chiral molecules.
Circular dichroism (CD), measured as a function of wavelength, is the difference in absorbance of left-handed circularly polarized light (L-CPL) and right-handed circularly polarized light (R-CPL). This difference can be detected when a molecule contains one or more light-absorbing groups - so-called chiral chromophores.