A light beam is extinguished by a pair of crossed polarizers. A sugar solution is placed between the polarizers and the beam appears once more because the plane of polarization has been rotated by optically active molecules in the solution.

High intensity light projector (part of equipment)
A pair of plastic polarizer discs
Wood block for holding the sheets vertical
Solution cell (a plastic plumbing T joint ca. 2 1/2" in diameter and 5" long with glass plates glued to the end so it holds water...part of the equipment)
A color filter (green is best, unless a narrow band pass filter can be found; several come with the equipment)
Paper clip
Test solution [500 ml of 38 weight % glucose is good]

The apparatus is shown below. The wooden block supplied with the equipment has grooves cut in it to hold the polarizer discs; it also supports the solution cell. The projector supplied with the equipment is the light source shown. Tape the color filter to either end plate.

Shine the light on the wall and adjust so that the filament is in focus, if possible. Adjust the position of the solution cell (T joint) so that the light shines through it. Place the first polarizer in the groove at one end of the block so that light passes through the solution and the polarizer, and observe the effect. Place the second polarizer in the other groove. Rotate one of the polarizers to extinguish as much of the light projected onto the wall as possible. Mark the orientation of the rotating polarizer by placing the paper clip at the top point. Pour the test solution into the T joint. If it is optically active, it will rotate the plane of the polarized light formed by the first polarizer, and the light image will reappear on the wall. Rotate the polarizer until the image is extinguished again. The amount of rotation is equal to the amount the plane of the polarized light was rotated as it passed through the test solution.

The demonstration above is used to detect optical activity and to measure the amount of rotation. A beam of light cannot pass through a pair of crossed polarizers, because the first polarizer allows only light with a certain polarization to pass while the second only allows passage of light with the opposite polarization The substance or solution placed between the polarizers causes the plane of polarization to rotate, so that the second polarizer is no longer exactly lined up to absorb all the light emerging from the solution. The more concentrated the solution and the longer the path length of light going through the solution, the greater the rotation.

The glucose solution suggested under MATERIALS will rotate the plane of polarized light about 28°for each inch of pathlength, which means that a pathlength of about seven inches will appear to have no rotation at all. The procedure requires a well darkened room.

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