227P h y s i c s E d u c at i o nMay 2010

F r o n t l i n e

Various wave optics experiments can be carried out using an ordinary compact disc. The CD is suitable for use as a diffraction grating [1, 2]. For instance, a standard CD (700 MB) has 625 lines/mm.

Here we describe two white-light diffraction demonstrations for a large audience, realizable using a CD (as reflection or transmission grating) and a slide projector.

To obtain high-quality diffraction patterns we propose using a circular slit (figure 1) in the slide projector in place of a slide, rather than the simple linear slit used with linear gratings. The slit should be 15–20 mm in diameter and 1–3 mm wide. These dimensions are not critical. The preparation of a quality slit with relatively smooth edges can be done using a PC. The slit is first drawn by com-puter then it is cut by a plotter on a black (opaque) adhesive sheet. The sheet is then stuck onto a glass slide plate and, only after this, the ring band of the slit is removed carefully.

Mount the CD in a holder close to the projector objective (figure 2). Direct the light cone from the lens onto the working surface of the CD. Rotate the CD to a certain angle so that the reflected light beam passes above the projector and then falls on the screen. The circular slit is focused on the screen by means of the objective. A wonderful bright diffraction pattern (figure 3) can be seen on the screen.

o P t i c s

White-light diffraction with a CD

Figure 1. The circular slit.

Figure 2. Basic set-up for the experiment with the reflective CD.

Figure 3. The central part of the diffraction pattern from the reflective CD.

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228 P h y s i c s E d u c at i o n May 2010

F r o n t l i n e

This is a most impressive and attractive white-light diffraction demonstration for a mass audi-ence. The diffraction pattern can be shown even without a screen. It can be projected directly onto a wall or ceiling. The whole picture can easily be more than 10 m in diameter. We have projected it onto the ceiling of a big auditorium with a capacity of 200–300, to the great appreciation of everyone

in the audience. Darkening the room is important to obtain good visibility of the pattern.

It is also possible to demonstrate diffraction from a transmission grating using CDs. They are sold in large packs in which sometimes protective uncoated transparent discs without a metallized layer are placed above and below the metallized ones. These discs are considered defective and dis-posable but they still have the pre-stamped spiral lines and can be used as transmission gratings.

Using the same projector and circular slit as in the previous experiment, focus the slit on the screen. Mount the disc in a holder and place it close to the projector objective, so that the light cone from the lens falls onto the disc symmetric to its centre (figure 4). A colourful diffraction image of the projector lamp’s spectrum appears on the screen (figure 5). Thorough darkening of the room is even more important for this demonstration.

References[1] Gluck P 2002 Compact disc optics Phys.

Teach. 40 466 [2] Knauer T 2002 A compact disc transmission

spectroscope Phys. Teach. 40 466

Dragia Trifonov Ivanov and Stefan Nikolaev Nikolov Faculty of Physics, Plovdiv University, Bulgaria (e-mail draiva@uni-plovdiv.bg, stnikolov@uni-plovdiv.bg)

Figure 4. The basic set-up for the experiment with the transparent disc.

Figure 5. Diffraction pattern from the transparent CD.

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