overhead projector demonstrations Polarization of Scattered Light Linda L. Munchausen Southeastern Louisiana University Harnrnond, LA 70402 One of the more popular demonstrations of a physical phenomenon is the demonstration ofthe Tyndall Effect ( I 3).An exciting aspect of the demonstration is that the scattered blue light is plane polarized. Demonstration of the polarization effect can be used both to emphasize the relationship between chemistry, physics, and geology and to explain how Polaroid sunglasses work. Demonstration Create a colloidal dispersion either by reacting a solution of thiosulfate with acid ( I ) or by slowly adding milk (2)to a beaker of water with a n eyedropper. To make the demonstration effective for large classes, use a n overhead projector a s the light source. Create a thin beam of light through the colloidal solution by covering the stage of the projector with a piece of cardboard with a small circular hole (1-2 cm diameter) and placing the beaker containing the colloidal disoersion over that hole. The transmitted light can be viewed on a screcn and shown to change from yellow t~ red with incrrasinr concentration ofthe colloidd particles. The scattered light appears a s a pencil-thin, vertical beam arising within the solution from the hole in the cardh a r d . viewid through a pair of..blue-blocker" sunglasses, thc bcam of'scatterrd light will almost disapp~ar.View thc beaker through the lensfrom a pair of polamid sunglasses by holding the lens between the students and the beaker. When the lens is held in the same orientation a s it would be in the sunglasses, the beam of scattered light can be seen on each side of the lens but not through the lens. Rotate the lens 90" and the scattered light remains visible through the lens. A thin crystal of the mineral tourmaline is a natural polarizer and will exhibit the same behavior. Discussion As long a s the diameter of the colloidal particles is small compared to the wavelengths of visible light, the intensity of the scattered light varies with (l/h4).Thus, light of the smaller wavelength (blue) is more efficiently scattered, while the red light is transmitted to the screen. Although s t u d e n t s sometimes perceive t h e scattered light a s "white", it is clearly blocked by "blue-blocker" sunglasses. According to Brewster's law, polarization of reflected light is complete when the plane of polarization is perpendicular to the plane of incidence (4). Thus, the scattered light can be shown to be plane polarized when viewed through a polarizing lens. Man-made polarizers, such a s those used in the manufacture of sunglasses, can be created from charged molecules with a long axis, which will align themselves with a strong electric field and can then be sandwiched between layers of plastic. The same type of molecular alignment is produced in nature during the crystallization of the mineral tourmaline. The use of blueblocker and Polaroid sunglasses leads easily into discussions of the purpose and function of sunglasses and the na-
edited by
DORISKOLB Bradley Univenity
Peoria, lL61625
ture of glare, which is scattered light. I n addition, the use of a tourmaline crystal can lead to discussions of crystal lattice and the application of chemistry and physics to geology Literature Cited 1. Jenkms.FA.;White. H. E. Fundnmn(ois ofopties, 3rd ed.; McGrc~w.Hfi1:NewYark, 1957:pp 505-507. 2. Alyea, Hubert N.; D u r n , Rederic B.. Ed. ~stodhonstmtionsinCkamisfr).;Jour nal of Chemical Education:Esston. PA. 1969: p 3. Amery, H. The Know How Book ofE=perimnfs; E. M. C. Corparation: Sf. Paul, MN,
169.
1978. 4. Jones, E. R.; Childem, R. L. Conbmpomry College Physics;Addison-Wesley : New Ywk, 1990: pp 686688.
A Digital Scientific Calculator for Overhead Projection Donald 8. DuPr6 University of Louisville Louisville, KY 40292 Some pocket scientific calculators with liquid crystal displii~d( L ~ D Smay , bc modified for d!spla? ofcalcu~atwnion the overhead vrolcctor. Many LC& are constructed w t h a piece of metailictape on the rear, which normally acts to reflect light back toward the observer. This tape may be peeled off to allow transmission of light and thus projection of the digits on a screen. Several commercial calculators that we have examined do not have interfering circuitry behind the display and are suitable for modification a s described below for the Tozai Model ATC 915 Scientific Calculator' and the Casio Model k-250Hs Student Scientific Calculator (Large Display). (Unfortunately, the Texas Instruments calculators t h a t we have examined have obstructing components behind the LCD.) To modify the Tozai calculator, f r s t remove two screws a t the top of the back panel. Pry off the back, which is also held in place by two tabs on the inside bottom. Carefully push the two wires to the batteries outwards and to the side of the display. Remove and discard the foam pressure pad adhering to the top of the LCD. Carefully peel back the white, mirror tape on the rear of the LCD package. The residual adhesive on the glass may be removed with acetone. (A gummy resin will form that may be rubbed off.) Cut out a 2- x 6-cm rectangular slot from the back of the plastic case to expose the display to transmittedlight. (The slot may be readily cut by first drillingseveral U4-in. holes in the case and then trimming the plastic with small wire cutting pliers or a razor blade knife.) Reassemble the case. A protective rear window may be cut from plastic film (such a s that used for overhead transparencies) and taped across the open slot. To modify the Casio calculator, first remove seven screws from the rear of the case. Pry off the back to expose the LCD. Set aside the batteries. The LCD in this calculator is held down with a rectangular piece of aluminum with a n adhesive on the side facing the display. Pry up this metal 'Dstrlo~tedby East West Dstr~o~t ng Co., Deerf~eoIL60015ana ava lao e at Wa greens DrJg Stores Volume 71
Number 2 February 1994
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strip and remove it, taking care not to put pressure on the LCD or tear the fragile film holding the electronic circuitry. Then peel off the metallic tape that wvers the rear of the LCD. For better visibility, the residual adhesive on the glass may be removed with a n ammonia based glass cleaner. (A gum also forms that may be ~ b b e doff with some effort. j Tack down the LCD with small pieces of tape or a n adhesive around the edges. Cut a rectangular slot in the rear plastic case to allow transmission of light. Replace the batteries in the proper orientation, reassemble the calculator and fashion a protective rear window out of transparency film. The clarity of the image projected fmm some LCDs is enhanced with polarized transmitted light. You may wish to experiment with polarizing film placed over the rear window of the calculator a t the optimum orientation.
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Journal of Chemical Education
The digits of these displays may be further magnified with a Fresnel sheet magnifie? held in place on a wire stand about 4-5-in. above the calculator. (The exact height depends on focusing characteristics of your overhead pmjector.1 The pmjected images of these modified, inexpensive calculators compare well with commercial educational mode l ~but, , ~ of course, the keys do not show.
'Available from Edmund Scientific Co.. 101 E. Gloucester Pike. ~. Barr nglon, Nu 08007. 'See, for example, the Eaucator Sclent~flc Overnead Calc~ator from Central Scenllf'cCo.,3300 CEhCO Parkway, Frankl n Par*. IL 601 31 ~
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