Sodium atoms polarized light

Thus there are two classes of optically active crystals. One, like quartz and sodium chlorate, only shows the effect in the crystalline state. Here the ability to rotate the plane of polarized light is related to the fact that the atoms in the crystal are arranged as right-handed or left-handed spirals or other asymmetric shapes. The second class of optically active crystals contains molecules or ions, such as certain tartrates, that are themselves asymmetric the effect persists even when the crystal melts or is dissolved. More details on this subject are given in Chapter 14. 

Molecules which do not have a center, a plane or a reflexion-rotation axis of symmetry, are optically active. The best example is the asymmetric carbon atom which has four different substituents. These molecules have some rotatory power, which is routinely measured in polarimetry by their ability to rotate monochromatic light (most often the D line of sodium at 589 run or 5893 A) after preliminary passage through a polarizing crystal. 

---