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Lab Phys-112 3: Mirrors and Lenses Objective: The purpose of this lab is to investigate image formation and properties of concave mirrors and lenses. Several different methods will be used to measure the focal length of the mirrors and lenses. Part I: Concave Mirror 1. Acquire a light source with power supply, an optics bench, a concave mirror and a half-screen. Place the light source at one end of the optical bench, with the image arrow arranged to shine down the bench. Place the mirror about 10 cm away from the light source. Place the half-screen between the mirror and the light source. You will focus the image from the mirror onto the half-screen. Adjust the position of the half-screen until the image is sharply in focus on the screen. Read do, the distance from the object to the mirror and d i, the distance from the image to the mirror, as accurately as possible. Use a meter stick to measure the width across the object arrow. This will be ho. Measure the width of the image arrow; this will be hi. Record these values in a table like the one below. Trial 1 2 3 3. do (cm) di (cm) ho (cm) hi (cm) f (cm) m1 m2
Throw the image out of focus by moving the mirror about 20 cm farther away from the light source. Have another member of the lab group move the half-screen so the image is in focus, and remeasure do and di, ho and hi. The image height may be difficult to measure since it is so small, but do your best. Repeat Step 3 to obtain a third measurement of do and di, ho and hi. For each trial use d o and d i to calculate the focal length of the mirror. For each trial calculate the magnification of the mirror using do and di, and also by using ho and hi. Record these three values in the table. Which do you think is the more accurate measurement for the magnification? Explain below your table.
Part II: Ray Box 1. Remove the light source from the optics bench to use it as a ray box. Acquire a protractor and a three-sided ray box mirror. Place the ray box, label side up, on a white sheet of paper in someone's lab notebook so that it shines onto the plane mirror surface of the three-sided mirror. Adjust the box so one white ray is showing. Draw a line in your notebook along the plane mirror surface. Adjust the ray box so it hits the mirror at an angle, allowing you to see the incident and reflected ray. Trace the rays. Then use the protractor to construct a normal at the point of incidence. Measure and label the incident and reflected angles. Repeat for everyone's notebook in your group. Adjust the ray box so it produces five white rays. Shine the rays onto the concave mirror so light is reflected back toward the ray box along the mirror axis. Trace the surface of the mirror and the incident and reflected rays in your notebook. Indicate the incoming and outgoing rays with arrows in the appropriate directions. Measure and label the focal length of the concave mirror. Repeat for everyone's notebook. Repeat the previous step with the convex 3. mirror.
Part III: Converging Lens 1. Remove and return the concave mirror and half-screen, replace the ray box in the optics bench as a light source, and acquire a converging lens in a lens holder and a viewing screen. Using the converging lens in the lens holder, go into the darkened hall and focus a distant light source (the window at the end of the hall or electric light) on a piece of paper. Use a meter stick to measure the distance from the lens to the paper in centimeters. Record this value in your notebook as the focal length of the lens. Return to the lab room. On the optical bench, position the lens between the light source and the screen. Set the screen as far away from the light source as possible and move the lens so an enlarged image is focused on the screen. Measure the image distance (distance from lens to screen), object distance (distance from lens to light source), image size, and object size. Record in a table like the one below: Trial 1 2 3 4 Etc. 3. 4. do (cm) di (cm) hi (cm)
Move the lens to a second position where the image is in focus (do not move the screen or light source). Now the image will be reduced. Measure do, di and hi for this position also. Move the screen toward the object until there is only one image that can be focused. Then move the screen a few centimeters farther away from the object so two focused images can again be obtained. Repeat the previous procedure, finding do and di for the reduced and enlarged images, but don't measure h'. Repeat these measurements for 2 other intermediate positions of the screen so that you have eight pairs of d o and d i data. Try to split the difference between your previous positions to get a good data spread. Use Vernier Graphical Analysis to make a plot of 1/ do vs. 1/ di. You should obtain a straight line, and the x- and y-intercepts are equal to f. Use the computer to measure these two values, average them, and write the result in your lab notebook. Print the graph for everyone.
Questions 1. A plane mirror essentially has a radius of curvature of infinity. Using this fact, along with the mirror equation, show analytically that (a) the image of a plane mirror is always virtual, (b) the image is "behind" the mirror the same distance as the object is in front of the mirror, and (c) the image is always upright. Explain what characteristics make convex spherical mirrors applicable for store monitoring and concave spherical mirrors applicable as flashlight reflectors. For the first two lens measurements you made in Part III, calculate magnifications using d o and di, and then using ho and hi. How do these values compare? How does the focal length you measured directly in Part III compare to the focal length obtained through graphical analysis? Which do you think is more trustworthy? Why? Use the thin-lens equation to explain why, for a given screen-object distance, there are two positions where the image is in focus.
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