Optical rotation (optical activity) is the turning of the plane of linearly polarized light about the direction of motion as the light travels by certain materials. It happens in solutions of chiral molecules such as sucrose (sugar), solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms or molecules. It is used in the sugar industry to measure syrup concentration, in optics to manipulate polarization, in chemistry to characterize substances in solution, and in optical mineralogy to help identify certain minerals in thin sections. It is being developed as a method to measure blood sugar concentration in diabetic people.