Pharmaceuticals that are optically active have a molecule that is their mirror image. The optically active pharmaceutical and its mirror image are referred to as enantiomers and while they might possess identical physical properties, their physiological properties can differ dramatically. Enantiomers can have different pharmacological effects, toxicity, or have no effect at all. In order to ensure the correct pharmacological effect of a drug, quality control includes testing for enantiomers. Enantiomeric purity is controlled by measurement of optical rotation, which is dependent on the wavelength. Polarimeters determine the optical rotation of optically active substances and are essential in drug quality control to ensure the enantiomeric purity and therefore efficacy of pharmaceutical drugs.