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For a pure substance in solution, if the color and path length are fixed and the specific rotation is known, the observed rotation can be used to calculate the concentration. This usage makes a polarimeter a tool of great importance to those trading in or using sugar syrups in bulk.
In the presence of magnetic fields, all molecules have optical activity. A magnetic field aligned in the direction of light propagating through a material will cause the rotation of the plane of linear polarization. This Faraday effect is one of the first discoveries of the relationship between light and electromagnetic effects.
Optical activity or rotation should not be confused with circularly polarized light. Circularly polarized light is often presented as a linear polarization rotating as the light propagates. However, in this picture, the polarization completely rotates in a length equal to the wavelength (roughly one micrometer for visible light), and it can happen in vacuum. In contrast, optical activity occurs only in a material, and a complete rotation occurs in a length of millimeters to meters, depending on the material.
Read more about this topic: Optical Rotation
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