What are Optical Isomers?

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  • Written By: Kathryn Pless
  • Edited By: J.T. Gale
  • Last Modified Date: 05 November 2019
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Two molecules of the same chemical makeup may be constructed with different atomic configurations. Atoms in these molecules are arranged differently, and when their mirror images are turned to face the same direction, the atoms in these molecules do not line up with each other. These molecules also are known as chiral molecules. Chiral molecules cannot be superimposed upon each other. An isomer is such a molecule.

An optical isomer is a molecule that has an affect on plane-polarized light. Light waves have specific frequencies. Polarized light travels in waves on a specific plane, or frequency of vibration. When a single frequency of light travels through optical isomers, its plane is rotated. This rotation can be clockwise or counter-clockwise depending on the electrical charge of the isomers.

Scientists usually test for optical isomers using a polarimeter. Components of a polarimeter usually include a piece of polarized material through which light passes, a tube used to hold a solution, a rotatable analyser, and an eyepiece. The tube is filled with water, which has no electrical charge, and the analyser is rotated so that no light can be seen passing through it.

The polarizers usually are crossed at a 45 degree angle. A solution containing optical isomers is added to the water and the analyser is turned until light can be seen coming through the instrument. In this way the effect of the isomers can be measured and the angle of rotation can be noted.


Examples of isomers are amino acids, sugars, and proteins. The different tastes and smells of some foods can be attributed to the way the atoms in these foods are aligned. Receptors in the mouth and nose may perceive the taste and aroma of foods based on how these molecules are arranged and how they react to these receptors.

Optical isomers can be used to measure the purity of a chemical solution, as well. For example, they can be used in the sugar industry to measure the concentration of bulk sugar syrup. In medicine, the use of these isomers is being developed to measure the levels of blood sugar in diabetics. Optical mineralogy can use isomers to identify materials in a thin section of mineral deposits, as well.

The separation of optical isomers can be a useful tool for scientific research. In studying amino acids, these acids can be separated and measured by using optical isomers. A substance containing these isomers is passed through a solid or liquid that has electrically charged atoms, and the isomers are separated by the reaction of the electrical charge of the solution and the isomers. Optical isomers are a common occurrence in natural chemistry and can be used in many industries including medical research.


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