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Molecules of a single substance often have the same properties, but they can be oriented differently as if left- or right-handed. Light, polarized along a single direction, can pass through a structure such as a crystal, and be rotated in one direction or another. If they tend to scatter light this way, have the same characteristics, and seem to be mirror images of one another, these molecules are often called chiral enantiomers. Optical activity refers to how a chiral substance can rotate the polarization of light and make it dimmer. A polarimeter is an instrument that is often used to measure this activity, using a sample of the material to test its optical activity first hand.
The molecular structure of a substance typically determines the relationship between chirality and optical activity. Depending on the material, the plane of polarized light can be directed in a clockwise or counter-clockwise direction. Two identical molecules that affect light in opposite directions are called enantiomers. An equal amount of each usually cancels out the effect. If one chiral substance dominates the other, however, its properties will rotate light waves in the respective direction.
A polarimeter is often used to measure optical activity. The front filter polarizes incoming light which passes through a tube filled with the sample being measured. There is typically an analyzer at the end of the tube and past that is where the effect can be observed by the human eye. If the plane polarized light is rotated, then it is generally dimmer, while the effect can be counteracted by rotating the analyzing lens. The level of optical activity can be determined by using a formula the number of degrees this lens is rotated.
Optical activity is often measured in organic chemistry, typically in carbon atoms. It can also be seen in optically active compounds such as sugar and even glycine, an amino acid. There are two nearly identical forms of a molecule in each of these substances. Knowing which one is which requires experimentation, but is often important with pharmaceuticals. Some drugs for depression and arthritis have had adverse health effects in their chiral forms, so one form of the molecule needs to be filtered out.
Discovered in the mid-1800s, optical activity was first used in identifying crystals of tartaric acid in French wine. It is studied in modern times with regard to quantum mechanics, as well as electric and magnetic fields. Optical activity occurs in both organic and inorganic molecules.