In the Classroom
edited by
Overhead Projector Demonstrations
Doris K. Kolb
Using Overhead Projector to Simulate X-ray Diffraction Experiments
Bradley University Peoria, IL 61625
Veljko Dragojlovic† Department of Chemistry, Northwest Community College, 5331 McConnell Avenue, Terrace, BC, V8G 4C2, Canada;
[email protected] Several demonstrations described in this Journal employ lasers to demonstrate diffraction of light and illustrate X-ray crystallography experiments (1–6 ). A similar demonstration can be performed using a common overhead projector. The setup is shown in Figure 1. The glass surface of the overhead 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) spot was projected onto the screen. A white board made a good projection screen, as one could mark the position of spots on it. In the path of the light beam we placed a pattern made of a combination of transmission diffraction gratings (7).1 Thus, white light was dispersed into a spectrum and a diffraction pattern was obtained (Fig. 2). This diffraction pattern depends on the combination of diffraction gratings. We used a single diffraction grating, two diffraction gratings pasted together at an angle of 90°, three diffraction gratings pasted together at angles of 60°, and six diffraction gratings in a helical arrangement. In a dark room, and using relatively goodquality gratings, third-order diffraction spots were visible. A projection distance of 2.4 m gave an 80-cm diffraction spacing (first order, red band) on the screen. If the central spot of the diffraction pattern is perceived as too bright, it can be labeled with a black marker. As the third-order diffraction spot may not be immediately visible, covering and uncovering the hole in the projector cover should help students to locate its position. Since human eyes are most sensitive to green light, most students are able to locate the position of the green band of the third-order diffraction. This is a good place to introduce averted vision technique to students. To illustrate diffraction of a light beam of a single color (narrow band of wavelengths) a suitable filter can be used.2 Because the filters attenuate light, the third-order diffraction spots may no longer be visible. The light beam can be made visible by spraying water mist along the light beam from an atomizer such as an ordinary plant-leaf sprayer. Students can be asked what factors affect the observed diffraction pattern. From the observation that white light was dispersed into a spectrum of colors, they should be able to conclude that different wavelengths of light were diffracted by different angles. After some discussion, this should lead into the conclusion that using a single wavelength will allow
† Present address: Department of Math, Science, and Technology, Nova Southeastern University, 3301 College Ave., Ft. Lauderdale, FL 33314.
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us to determine the distance between the individual grooves of the grating. After the wavelength of diffracted light and the distance between the grooves were compared (λ = 400–700 nm for the visible light and d ≈ 4200 nm for the diffraction grating), students were asked to draw a conclusion about the Diffraction gratings covering lens
Carboard covering projector surface
Figure 1. Overhead projector with a mask and diffraction grating.
electromagnetic radiation wavelength necessary to obtain diffraction patterns of crystals. Given that the distances in crystals are of the order of 10 {10 m, the electromagnetic radiation wavelength is λ ≈ 10{10–10{11 m (X-rays). As a classroom activity (similar to one described in the ref 1) the spacing between the lines of a grating can be calculated if the wavelength of transmitted light is known.3 Alternatively, once the spacing is known, the wavelength of diffracted light can be calculated. The light beam should be orthogonal to the screen, and Fraunhofer’s equation for diffraction should be used as described in ref 2. For this activity it is the best if there is a square hole on the projector cover and the distance b is measured from the edge of the central (zero-order) square to the corresponding edge of the first-order diffraction square (Fig. 3). This activity is particularly suitable for high school students because it does not use laser pointers and therefore requires neither safety precautions nor extensive supervision.
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
In the Classroom
a
b
c
Figure 2. Diffraction patterns obtained with (a) blue filter, (b) red filter, and (c) without a filter.
Notes 1. Replica diffraction gratings mounted on a 35 mm slides were obtained from Edmund Scientific Company, 101 E. Gloucester Pike, Barrington, NJ 08007-1380, stock # C39,502. These slides were of uneven quality. Out of a package of 25 slides, only six were of satisfactory quality for this demonstration. Diffraction grating sheets provided by the same supplier (stock # C40,267) were of better quality and the patterns for the demonstration were prepared by cutting the sheets to size and mounting them on a 6’’ x 6’’ cardboard frames. 2. Plastic color filters were obtained from Sargent-Welch Scientific 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. I would like to thank Don Hill from the Northwest Community College for recording the absorption spectra of the filters.
Screen
b
c
a φ Diffraction grating
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 diffraction gratings appeared in Grossman, W. E. L. J. Chem. Educ. 1993, 70, 741.
Figure 3. Diffraction experiment with transmission diffraction grating.
JChemEd.chem.wisc.edu • Vol. 76 No. 9 September 1999 • Journal of Chemical Education
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