In the Classroom

1240 Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu

Overhead Projector Demonstrationsedited by

Doris K. KolbBradley UniversityPeoria, IL 61625

Using Overhead Projector to SimulateX-ray Diffraction Experiments

Veljko Dragojlovic†

Department of Chemistry, Northwest Community College, 5331 McConnell Avenue, Terrace, BC, V8G 4C2, Canada;veljko@ocean.nova.edu

Several demonstrations described in this Journal employlasers to demonstrate diffraction of light and illustrate X-raycrystallography experiments (1–6 ). A similar demonstrationcan be performed using a common overhead projector.

The setup is shown in Figure 1. The glass surface of theoverhead projector was covered with a piece of cardboard,which had an 8-mm circular or square hole in the center.The projector was focused so that a circular (or square) spotwas projected onto the screen. A white board made a goodprojection screen, as one could mark the position of spotson it. In the path of the light beam we placed a pattern made ofa combination of transmission diffraction gratings (7).1 Thus,white light was dispersed into a spectrum and a diffractionpattern was obtained (Fig. 2). This diffraction pattern dependson the combination of diffraction gratings. We used a singlediffraction grating, two diffraction gratings pasted togetherat an angle of 90°, three diffraction gratings pasted togetherat angles of 60°, and six diffraction gratings in a helicalarrangement. In a dark room, and using relatively good-quality gratings, third-order diffraction spots were visible. Aprojection distance of 2.4 m gave an 80-cm diffraction spacing(first order, red band) on the screen. If the central spot ofthe diffraction pattern is perceived as too bright, it can belabeled with a black marker. As the third-order diffractionspot may not be immediately visible, covering and uncover-ing the hole in the projector cover should help students tolocate its position. Since human eyes are most sensitive togreen light, most students are able to locate the position ofthe green band of the third-order diffraction. This is a goodplace to introduce averted vision technique to students. Toillustrate diffraction of a light beam of a single color (narrowband of wavelengths) a suitable filter can be used.2 Becausethe filters attenuate light, the third-order diffraction spots mayno longer be visible. The light beam can be made visible byspraying water mist along the light beam from an atomizersuch as an ordinary plant-leaf sprayer.

Students can be asked what factors affect the observeddiffraction pattern. From the observation that white light wasdispersed into a spectrum of colors, they should be able toconclude that different wavelengths of light were diffractedby different angles. After some discussion, this should leadinto the conclusion that using a single wavelength will allow

us to determine the distance between the individual groovesof the grating. After the wavelength of diffracted light and thedistance between the grooves were compared (λ = 400–700nm for the visible light and d ≈ 4200 nm for the diffractiongrating), students were asked to draw a conclusion about the

Diffraction gratingscovering lens

Carboard coveringprojector surface

Figure 1. Overhead projector with a mask and diffraction grating.

†Present address: Department of Math, Science, and Technology,Nova Southeastern University, 3301 College Ave., Ft. Lauderdale,FL 33314.

electromagnetic radiation wavelength necessary to obtaindiffraction patterns of crystals. Given that the distancesin crystals are of the order of 10{10 m, the electromagneticradiation wavelength is λ ≈ 10{10–10{11 m (X-rays).

As a classroom activity (similar to one described in theref 1) the spacing between the lines of a grating can becalculated if the wavelength of transmitted light is known.3

Alternatively, once the spacing is known, the wavelength ofdiffracted light can be calculated. The light beam should beorthogonal to the screen, and Fraunhofer’s equation fordiffraction should be used as described in ref 2. For thisactivity it is the best if there is a square hole on the projectorcover and the distance b is measured from the edge of thecentral (zero-order) square to the corresponding edge of thefirst-order diffraction square (Fig. 3). This activity is particu-larly suitable for high school students because it does not uselaser pointers and therefore requires neither safety precautionsnor extensive supervision.

In the Classroom

JChemEd.chem.wisc.edu • Vol. 76 No. 9 September 1999 • Journal of Chemical Education 1241

Notes

1. Replica diffraction gratings mounted on a 35 mm slides wereobtained from Edmund Scientific Company, 101 E. Gloucester Pike,Barrington, NJ 08007-1380, stock # C39,502. These slides were ofuneven quality. Out of a package of 25 slides, only six were of satisfac-tory quality for this demonstration. Diffraction grating sheets providedby the same supplier (stock # C40,267) were of better quality and thepatterns for the demonstration were prepared by cutting the sheets tosize and mounting them on a 6’’ x 6’’ cardboard frames.

2. Plastic color filters were obtained from Sargent-Welch Scien-tific Company, P.O. Box 20060, London, Ontario, N6K 4G6; stock# 3662.

3. The absorption maxima for the filters from note 2 were 448,535, and ~660 nm for the blue, green, and red filter, respectively. Iwould like to thank Don Hill from the Northwest Community Col-lege for recording the absorption spectra of the filters.

Literature Cited

1. Hughes, E. Jr.; Holmes, L. H. Jr. J. Chem. Educ. 1997, 74, 298.2. Lisensky, G. C.; Kelly, T. F.; Neu, D. R.; Ellis, A. B. J. Chem.

Educ. 1991, 68, 91.3. Klier, K.; Taylor, J. A. J. Chem. Educ. 1991, 68, 155.4. Spencer, B. H.; Zare, R. N. J. Chem. Educ. 1991, 68, 97.5. Segschneider, C.; Versmold, H. J. Chem. Educ. 1990, 67, 967.6. Brisse, F.; Sundararajan, P. K. J. Chem. Educ. 1975, 52, 414.7. A detailed review on the manufacture and properties of diffrac-

tion gratings appeared in Grossman, W. E. L. J. Chem. Educ. 1993,70, 741.

Figure 2. Diffraction patterns obtained with (a) blue filter, (b) red filter, and (c) without a filter.

a cb

Diffractiongrating

Screenb

ac

φ

Figure 3. Diffraction experiment with transmission diffraction grating.