This demonstration can be used if the projector and special solution cells described in Demo-022 are not available. An overhead projector is used as a light source in this one.

A beaker or other transparent container at least 6 inches tall with a flat bottom.
A sheet of cardboard big enough to cover the overhead projector stage with a hole 1 inch in diameter.
A transparency with a line on it
Two plastic polarizer sheets, one with a line on it
A color filter (green, if possible)
Test solution [500 ml of 38 weight % glucose is good]
Overhead projector

Tape one of the polarizer sheets (the one without the line) over the front of the overhead projector head in such a way that it can be flipped out of the way. The color filter should be taped to the underside of the cardboard sheet. Tape the coardbord sheet to the projector stage so that the hole in it is exactly underneath the upper lens. Place the transparency on the cardboard, with the polarizer on top of that. Tape the transparency to the cardboard in such an orientation that when its line coincides with the line on the polarizer, the light is extinguished.

With the upper polarizer flipped up, show the colored light beam. Focus the projector on the line on the transparency. Flip down the polarizer, and explain that half the light has been extinguished because only the correctly polarized part can pass through the polarizer. Put the polarizer on the transparency, and turn it back and forth so that it can be seen that it extinguishes all the light at a particular angle. Turn the transparency to make the two lines coincide, setting the angle for full extinction. Flip the upper polarizer out of the beam to show that this is so. Tape the lined polarizer to the bottom of the container, and place the container over the aperture. Flip down the upper polarizer, and make any fine adjustments to mazimize the extinction. Add the test solution to the container, and the light appears once more on the screen. Turn container and the lined polarizer to achieve maximum extinction once more. Flip up the upper polarizer. The amount of rotation of the lower polarizer is the amount the sugar solution rotated the plane of the polarized light.

Many molecules that result from biological processes are dissymmetric, that is, they differ from their mirror images (as your right hand differs from your left). Such a molecule has the property of rotating the plane of polarization (the direction of the vibrating electric field vector) of light when light passes through it or a solution of it. This is called optical activity. 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 (say up-and-down, or north-and-south in the case of the overhead projector set-up) to pass while the second only allows passage of the other (sideways, or east-and-west), which the first removed. 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 instruments which make this kind of measurement are called polarimeters.

None

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.

PROCEDURE II is easier to prepare, but more complicated to present. The stage always appears bright, but the effect of adding the sugar solution is less dramatic. Viewers must be able to see the overhead projector stage to understand what is happening. While a beaker works reasonably well, a better container can be made by cutting some 1½" i.d. tubing (glass, PVC, plexiglass) to a length of six to ten inches, sawing one end as flat as possible, and gluing it to a piece of flat glass 2-4" square. Rotating the container and polarizer is difficult, because it tends to drift off center, and the presenter must look at the aperture on the projector stage rather than the screen. The color filter will get quite hot if the projector is left on too long.