Forrest J. Frank
and Sharon M. Kidwell lllinois Wesleyan University Bloomington, Illinois 61701
Polarimeter for an Overhead Proiector
The widespread use of overhead projectors makes an inexpensive, easy to make polarimeter desirable. Described here is a polarimeter costing less than $5. The polarimeter tube consists of an inexpensive flashlight with the end, switch, and reflector removed. Epoxy cement is used to fasten the glass lens in place and to cement a piece of metal over the switch hole. To eliminate reflection, the inside of the tube is painted black. The base is made from I/&. plywood with a hole and slot sawed out. Three pieces of wood are positioned around the hole to hold snugly t,he lid of a jelly jar which has a hole in it the size of the hole in the plywood. A metal rod is attached to the jar lid to act as a pointer. Another piece of plywood, with a circle removed, is placed on the three blocks to serve as a platform on which to rest the polarimeter tube. Into the slot is placed either a large protractor or a scale drawn on a transparent sheet. The dimensions are not crucial and can be varied to fit any projector and any size flashlight. The aolaroid analvzer fits into the iar lid and consists of three pieces of polaroid as illustrated in Figure 1 (available from Edmund Scientific Company, 400 Edscors Building, Barrington, N. J., 08007). The arrows show the direction of polarization for each piece. Two half circles A and B are polarized at 90' to each other, and they are placed on a full circle C polarized a t 45" to A and B. A piece of tape is used as a hinge to attach a polaroid lid to the top of the polarimeter tube
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lournol o f Chemicul
Education
to allow for replacement in exactly the same position after filling. In operation (see Fig. 2), a red filter is placed on the projector stage under the hole in the base; the polarimeter tube filled with solvent is placed on the hole; and the pointer is moved until the two half circles are of equal intensity. In order to match the intensities, the projector lens is raised to its maximum height to improve the focus. The lens is lowered to bring the scale into focus for reading the angle of rotation. The tube is then filled with the optically active solution and the pointer rotated until equal intensities are observed. The difference in the above angles is the observed rotation. I n a dark room, angles are reproducible within 1-2" and observed rotations for 10 and 20% sucrose solutions agree within 1-2' of thereotical values.
Figure 1.
Polaroid onalyrer.
Figure 2. Polorimeter mounted on an overheod proiodor.
