Within a lecture on biological resolution, the synthesis of single enantiomers, and the naming and 3D visualization of omeprazole, Professor Laurence Barron of the University of Glasgow delivers a guest lecture on the subject of how chiral molecules rotate polarized light. Mixing wave functions by coordinated application of light’s perpendicular electric and magnetic fields shifts electrons along a helix that can be right- or left-handed, but so many mixings are involved, and their magnitudes are so subtle, that predicting net optical rotation in practical cases is rarely simple.