Physics 2020, Fall 2011 Lab 9 page 1 of 6 From Mc Dermott, Schaffer, & P.E.G. (University of Washington) Tutorials in Introductory Physics, Prentice Hall, Inc., First Edition, 2002 University of Colorado at Boulder, Department of Physics Circle your lab day and time. Your name: Tue Tue Tue Wed Th Th Th Fri TA name: 10-12 12-2 2-4 12-2 10-12 12-2 2-4 12-2 Lab 9: Mirrors PART I: The Method of Parallax A. Close one eye and lean down so that your eye is at table level. Have your partner drop a small piece of paper (about 2 mm square) onto the table. Hold one finger above the table and then move your finger until you think it is directly above the piece of paper. Move your finger straight down to the table and check whether your finger is, in fact, directly above the paper. Try this exercise several times, with your partner dropping the piece of paper at different locations. Afterwards exchange the roles. How can you account for the fact that when your finger misses the piece of paper, your finger is always either in front of the paper or behind it, but never to the left or to the right? B. Suppose that you placed your finger behind the paper (as shown at the right) while trying to locate the piece of paper. Predict whether your finger would appear to be located to the left of, to the right of, or in line with the piece of paper if you moved your head: to the left to the right Check your predictions and resolve any inconsistencies. C. Suppose that you had placed your finger in front of the piece of paper rather than behind. Predict whether the paper or your finger would appear on the left when you move your head to the left. Check your answer experimentally and write down your observation. D. Devise a method based on your results from parts B and C by which you could locate the piece of paper. Your method should include how to tell whether your finger is directly over the piece of paper and, if not, whether it is in front of or behind the piece of paper. Discuss your method with your partner, then test your method and describe it below. Check your method with your TA.Physics 2020, Fall 2011 Lab 9 page 2 of 6 From Mc Dermott, Schaffer, & P.E.G. (University of Washington) Tutorials in Introductory Physics, Prentice Hall, Inc., First Edition, 2002 University of Colorado at Boulder, Department of Physics We will refer to this method that you devised for locating the piece of paper as the method of parallax.Physics 2020, Fall 2011 Lab 9 page 3 of 6 From Mc Dermott, Schaffer, & P.E.G. (University of Washington) Tutorials in Introductory Physics, Prentice Hall, Inc., First Edition, 2002 University of Colorado at Boulder, Department of Physics PART II: Image Location At your place you should have a small mirror and a few nails and pencils. Place the mirror in the middle of a sheet of paper. Stand one nail on its head about 10 cm from the front of the mirror. We call this nail the object. On the paper, mark the locations of the mirror and the object (nail). A. Place your head so that you can see the image of the nail in the mirror. Use the method of parallax to position a pencil so that it is located in the same place as the image of the object nail. Mark this location on the paper. Is the image located on the surface of, in front of, or behind the mirror? Please explain below. Would observers at other locations agree that the image is located at the place you marked? Let your partner check your answer experimentally. B. Move the nail off to the right side of the mirror as shown. Is the image located on the surface of, in front of, or behind the mirror? Please explain below. PART III: Ray Tracing In the following experiments, you will determine the location of an object and its image by a different technique, called ray tracing. This technique is based on a model for the behavior of light in which we envision light being either emitted in all directions by a luminous object (such as a light bulb) or reflected in all directions by a non-luminous object (such as a nail). A. Turn the sheet of paper over (or obtain a new sheet of paper). Stand a nail vertically at one end of the piece paper. Place your eye at table level at the other end of the piece of paper and look at the nail. Use a straightedge to draw a line of sight to the nail, that is, a line from your eye to the nail. Repeat this procedure to mark lines of sight from three other very different vantage points and then remove the nail. How can you use these lines of sight to determine where the nail was located? What is the smallest number of lines of sight needed to determine the location of the nail?Physics 2020, Fall 2011 Lab 9 page 4 of 6 From Mc Dermott, Schaffer, & P.E.G. (University of Washington) Tutorials in Introductory Physics, Prentice Hall, Inc., First Edition, 2002 University of Colorado at Boulder, Department of Physics B. Turn the sheet of paper over (or get a new sheet of paper). Place the mirror in the middle of the sheet of paper and place a nail in front of the mirror (about 10 cm away). On the paper, mark the locations of the mirror and the nail. Draw several lines of sight from your eye to the image of the nail. How can you use these lines of sight to determine the location of the image of the nail? Use the method of parallax to determine the location of the image of the nail. Do these two methods yield the same location of the image (to within reasonable uncertainty)? C. (Did you mark the location of the mirror and the object nail?) Remove the mirror and the object nail. For the one eye location that you used in part B, draw the path that light takes from the object nail to the mirror. Draw an arrow head on this line segment to indicate the direction that light moves along that part of the path. On the basis of the path of light from the object nail to the mirror (drawn in part C) and the corresponding line of sight from your eye to the image of the nail (drawn in part B), predict the path that light takes after it is reflected by the mirror. Repeat the steps above for other eye locations used in part B. Use a protractor to formulate a rule that you can use to predict the path light takes after it is
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