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What Are Optically Active Materials?

Optical rotation or optical activity (sometimes referred to as rotary polarization) is the rotation of the plane of polarization of linearly polarized light as it travels through certain materials as shown below.

The materials that can do so are called optically active materails. Optical activity occurs only in chiral materials, those lacking microscopic mirror symmetry.

When we view the light head-on, some optically active materials rotate the electric field clockwise, called dextrorotatory. And some optically active materials rotate the electric field counterclockwise, called levorotatory.

Unlike other sources of birefringence which alter a beam's state of polarization, optical activity can be observed in fluids. This can include gases or solutions of chiral molecules such as sugars, molecules with helical secondary structure such as some proteins, and also chiral liquid crystals. It can also be observed in chiral solids such as certain crystals with a rotation between adacent crystal planes (such as quartz) or metamaterials.

Arago first discovered this phenomenon in quartz in 1811. Two different crystalline structures of quartz produce d-rotatory and l-rotatory behavior. The two crystalline forms are said to be enantiomorphs of each other.

The optical activity of quartz is associated with its crystal structure, as evidenced by the fact that neither molten quartz or fused quartz demonstrate optical activity.

In the case of many naturally occurring organic compounds such as sugar, tartaric acid and turpentine, optical activity is exhibited in the liquid state. This shows that the activity is associated with the individual molecules themselves.

Note:

Rotation of light's plane of polarization may also occur through the Faraday effect which involves a static magnetic field, however this is a distinct phenomenon that is not usually classified under "optical activity."
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