Investigation 10.3.1 088 CD and DVD Storage Capacity If you look closely at the surface of a CD, you will notice a long spiral track circling from the centre of the disc out toward the rim. This track forms a reection diffraction grating. The angular location of each maximum depends on the spacing between slits and on the wavelength of the light source. In this investigation, you will use a CD as a diffraction grating and record where its maxima fall. You will then use that information and the known wavelength of the laser light to determine the groove spacing of the CD. You will then repeat the investigation using a DVD to determine the difference between the two. and use this information to assess which disc can hold more data. Purpose To determine whether a CD or a DVD can hold more data Equipment and Materials . class 1 or 2 laser or LED - scissors or other cutting device - ruler or other measuring device - clamps for mounting the screen - thick cardboard or pegboard - grid paper mounted on a piece of thick cardboard or pegboard - glue - one CD and one DVD Procedure 1. Predict what you expect to observe regarding the relative spacing of the CD and DVD tracks. 2. Create a data table in which to record your observations. 3. Set up the laser and the screen as shown in Figure 1. Set up the cardboard to be perpendicular to the laser beam and 10 cm from the screen. 4. Glue a CD to the cardboard. Test that the CD is glued rmly in position. screen Figure 1 The CD is mounted perpendicularto the laser beam and parallel to the screen. NEL ERVATIONAL STUDHf SKILLS MENU v Questioning v Plannsng 0 Observing . Hosoarcimg - Controlling 0 Analyzing . vaothesizrng Variables 0 Evaluating 0 Predicting 0 Perlorming 0 Communicating 5. Direct the laser light through the hole in the screen onto the mounted CD. The beam should strike the CD toward the outer edge, not closer to the middle. The light is incident perpendicular to the CD. The grooves on the CD act as a diffraction grating to produce a reection interference pattern on the screen. 0 It the laser uses 120 1.! electricity. to unplug it. pull on the plug. not the cord. Do not look into the laser light. Do not point the laser at anyone. 6. Get your teacher's approval for your setup. The interference pattern produced should clearly show the central maximum and secondary maxima. 7. Measure the distance from the central maximum to the rst maximum (x1), and from the central maximum to the second maximum (x2). Record your data. 8. Use the equation )t w: xi :61 W21. x2)2 + x; \\/(2L x1): + x? and your measured data to determine the spacing of the grooves on the CD. Show your calculations. 9. Repeat Steps 5 to 8 using the DVD. 10. Record your qualitative observations of the differences between the groove spacings on the CD and the DVD. Analyze and Evaluate (a) Let or represent the unknown spacing between the CD or DVD grooves and 91 and 92 represent the angles of the rst and second minima. Draw a diagram and label these quantities and the path difference between light from successive slits. la (b) How many maxima did you observe on each side of the central maximum in the CD interference pattern? The DVD interference pattern? all: I: (c) Compare the DVD and CD measured track separation, and discuss what that comparison implies about which one stores more data. IE! Apply and Extend (d) Explain how the spacing of the grooves on the CD needs to be different to observe more maxima. How would the wavelength need to be different instead? Is the same true for the DVD? I! III Chapter 10 Investigations 547