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LAB 11: FOCAL LENGTH OF LENS Equipment: Optical Bench 1 Light Source 1 75 mm converging lens 1 Crossed Arrow Target 1 Viewing Screen 1 Component Holders 2 The focal length of a spherical lens may be determined by use of the Fundamental Lens Equation. 111pqf. The magnification of the image is given by the equation iohqMhp . Procedure: 1. Set up the equipment as shown in the figure. Place the object, the crossed arrow target, on the front of the light source. 2. Put the lens 500 mm from the light source, ie. Object distance p=500 mm. 3. Move the viewing screen back and forth until the image is in focus. Measure the image distance and the image height. 4. Record the image distance, q, and height, hi, in the data table. Calculate the focal length and magnification, and record these. 5. Repeat for other object distances. 6. Find the % Difference between the experimental and theoretical focal lengths. Data Table: p(mm) q(mm) hi(mm) 1/p+1/q f(mm) hi/ho-q/p %Difference 500 450 400 350 300 250 200 150 100 75 50
Questions: 1. Find the average focal length, and the % Difference between the average and the theoretical focal length of 75 mm. 2. Why could you not obtain a focused image for some object distances? 3. What would the object distance need to be in order to obtain a magnification of -1? 4. For a converging lens of focal length f, where would you place the object to obtain an image as far from the lens as possible? 5. If you have a focused image for a converging lens of a slide projector and you move the viewing screen toward the lens, which way should you move the lens in order to restore the image to a sharp focus? Explain.