Using Crystal Birefringence to Study Molecular Recognition

That a growing crystal can discriminate between almost equally sized bromine atoms and CH3 groups can easily be demonstrated by light microscopy. Local birefringence of slightly impure crystals reflects their mechanism of growth. Merely a pair of polarization filters and a sensitive violet filter are required for the detection of the color effects (blue/yellow). Upon rotation through 90°, e.g., a crystal growing from the center shows the color change indicated on the right.