Measuring optical activity means measuring the optical rotation of a compound using a polarimeter, a device which employs polarimetry.

But why is this technique important? In chemistry, polarimetry is essential in understanding chiral molecules, which are characterized by their ability to rotate a plane of polarized light.

A chiral substance is going to have the ability to slow light down, visually altering the direction of oscillation. The wavelength’s change in direction is then measured.

Measuring the change in direction of a wavelength is possible because light constantly oscillates across all possible planes perpendicular to its origin.

For instance, a light bulb will light up a room because every wave emitted vibrates in its own direction; up and down, side to side, and perpendicular to the direction the light wave is traveling. This is known as unpolarized light.

In the case of polarimetry, light is polarized using a polarizer, an optical filter that allows for a specific wavelength to pass through, all while blocking other wavelengths of light. Through this process, an undefined or mixed light beam becomes a well-defined one, traveling in a single direction.

Polarimeters analyze different chiral solutions and their specific rotations by introducing linear plane-polarized light through a small tube containing a liquid sample. If a compound in a solution is optically inactive, the polarized light will not change.

However, if the compound is an optically active substance, the plane of light will change. As the light travels through the liquid sample, it will interact with the chiral molecules. This interaction causes the light to turn slightly. As it reaches the other end of the tube, its angle of rotation will be different than how it entered.

Polarimeters are used in numerous settings, from the pharmaceuticals, chemicals, and medical industries to the food industry.

The pharmaceutical industry relies on them in tandem with high-performance liquid chromatography systems to measure concentration of optically active compounds found in pharmaceutical drugs; the chemical industry uses polarimeters for characterization and identification of various compounds; the medical field relies on polarimetry for research in enzymology and toxicology; and lastly, the food industry uses polarimeters for quality control of products at a number of stages, as well as the concentrations and purity of food products.