Department of Physics & Astronomy at the University of Edinburgh

Optical Activity

The plane of vibration of plane-polarised light rotates continuously as it passes through an optically active substance. Some substances rotate the plane in a clockwise direction, dextro-rotatory - when viewing the light coming towards the observer - and other substances rotate the plane of vibration anti-clockwise, levo-rotatory.
In the case of solids, optical activity is normally associated with the crystalline structure; in the case of liquids, it is associated with the molecular structure. Crystal quartz, for example, has two crystal structures, one the mirror image of the other. Both structures are optically active but one structure is dextro-rotatory and the other is levo-rotatory.
Sugar solution, turpentine and tartaric acid are all optically active liquids. The rotatory power of liquids is associated with the structure of the individual molecules. Obviously, these molecules must be anisotropic and have some helical property, all molecules having the same direction of twist. It should be noted that a helix has the same direction of twist when viewed from either end.
To view the molecular structure of one of the sugars, sucrose, click here.


The image above shows the appearance of a plane polarised laser beam paasing along the axis of a tube filled with corn syrup. The plane of vibration of the laser beam is horizontal when it enters the syrup and it rotates through 1800 in about 15 cm.


Last Updated Friday, June 22, 2000 at 10:11
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