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1Diffraction and Compact DiscsPHYS 1301 F98Prof. T.E. CoanLast edit: 8 Aug ‘98IntroductionYou all have some experience with the phenomenon of diffraction, even if theword is new to you. In today’s lab you will use diffraction to probe the structure of thebottom of a compact disc (CD). Although an understanding of how a compact discworks is certainly not necessary to understand the important physics ideas of thetwentieth century, it is useful for you to understand some general features ofdiffraction and it is useful for you to have some understanding of how familiar deviceswork.A CD has a sandwich structure. The very bottom layer is made frompolycarbonate plastic to provide stiffness to the CD and to provide a surface to buildsmall aluminum structures on. These structures have a ridge-like geometry whenviewed from the side and have a common height, about 110 nanometers. The width ofthe flat or valley separating neighboring ridges varies and ranges from about 800nanometers to 3500 nanometers. On top of the ridge structure is lacquer to protect theridges, more plastic and then a label to identify the CD. Figure 1 is a schematic figureof a CD when viewed on edge.The bottom or unmarked surface of a CD has a spiral pattern of thesealternating raised areas and flat surfaces. Laser light is shot at the bottom of the CD,reflected and then detected by some electronics whose detailed workings we don’tneed to worry about. As the CD spins, the laser light illuminates various ridges andvalleys simultaneously. The reflected light undergoes some partial destructiveinterference so that the total amount of reflected light varies with time as the CDrotates. This varying amount of light is transformed by the CD player electronics into avarying sound signal that you hear as music.The ridges and valleys can be thought of as functioning together as slits in adiffraction grating. When we bounce light off of the CD bottom, the sets of ridges andvalleys will diffract the light and produce a characteristic diffraction pattern on ascreen. By measuring the geometrical features of the diffraction pattern, we can thenestimate the average size of the ridges and valleys.2PolycarbonatePolycarbonateRidgeCD top surfaceLaser lightFigure 1. Basic structure of a compact disc reflecting surface.Procedure1. Do not stare directly into the laser beam. Doing so will only hurt you, possiblyblinding yourself.2. Position the mirror cube, mirrored surface up, about halfway between the laserstand and the bookend. To start, about 20 cm or so should separate the laser standand the bookend.3. Aim the laser downwards and turn it on by connecting it to its power supply.4. Adjust the height and orientation of the laser diode so that when it is turned on thelight strikes the mirror and is reflected on the graph paper held by the bookend.Measure two heights ly and gy, the vertical distance the laser front end is abovethe mirror and the vertical distance the beam spot is on the graph paper,respectively. You will need also to measure two horizontal distances, xl and xg.These are the horizontal distances between the laser head and the mirror beamspot, and between the mirror beam spot and the beam spot on the graph paper,respectively. See figure 2 below.5. Change the horizontal distance by 10 cm or so between the laser diode and themirror. Move the stand with the diode, not the mirror. Position the screen so thatthe reflected beam spot strikes it. Record the new values of ly and gy.6. Repeat step 5 two more times. Measure and record again ly and gy , and xl andxg each time. You should have measurements for 4 different mirror setups.7. Remove the mirror. We now want to measure the diffractive properties of our CD.Tape the CD to a bookend so that it is positioned in a vertical plane. The CD’sunmarked side should face outwards. Separate the laser head from the CD byabout 10 cm or so and position it so that its axis is parallel to the plane of the table.Point the laser to the CD and set its height to match that of the CD center. Attachgraph paper to 2 book ends and position the bookends about the laser, one on eachside of the laser. Arrange matters so that the plane of the CD is parallel to the3planes of the pieces of graph paper and that both pieces of graph paper are in thesame plane. You may have to slightly adjust the CD-laser separation. See figure 3below.8. Turn on the laser. You should see 2 bright spots on each piece of graph paper. The4 bright spots may or may not lie in a horizontal line. Measure the distancebetween the laser head and the position of the two bright spots on the graph paper.The clever way to do this is to measure the distance between the two bright spotsand divide by 2. (Do you see why?) Next, measure the distance between the laserhead and the beam spot on the CD. You now have enough information tocalculate the angular position of the maxima on the graph paper. Calculate theangular position of each of the so-called “first order” maxima and average theirvalues. If you indeed have 4 maxima on your graph paper, repeat this procedureand measure the angular position of the “second order” maxima. Average this pair.You now have information to calculate the typical feature size of a CD.Figure 2. Setup for mirror measurements.CDScreenLaserFigure 3. Setup for CD measurements.Analysis (Be sure to show your calculations. Use another piece of paper.)1. Fill in data table 1 to compute the so-called incident angle incθ and the reflectedLaserScreenMirror blockLight path4angle θrfl in data table. You will need to compute the inverse tangent for a righttriangle using your measured distances found in the table.2. Complete data table 2. You need to remember the simple formula fordiffraction from a diffraction grating. Recall from a previous lab, dnsinθλ=, where dis the spacing between slits, θ is the diffraction angle, n is an integer ( n = 0,1,2,3…)which labels the so-called order of the bright spot, and λ is the wavelength of the lightpassing through the grating. Since the only unknown is this equation is the slit spacingd, you can easily solve for it. Although a CD is not a diffraction grating in the strictsense of the word, the CD acts similar to a diffraction grating because the reflectedlight undergoes constructive and destructive interference due to the special geometryof the CD’s bottom surface. What made the diffraction grating produce thecharacteristic pattern of alternating bright and dark


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SMU PHYS 1301 - Diffraction and Compact Discs

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