Lab & Pre-lab #11

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Lenses and Images v 0.1 Light Source at Focal Point f Focal Length Figure 2: Light rays starting at the focal point come out parallel lamp Figure 3: Arrangement of light source, F-diaphragm, and lens. Step 3: Mount the 15 cm lens on the optical bench using a lens holder. Do not tighten the mounting screw on the holder-leave it loose so that you can slide the lens back and forth. Step 4: Turn the optical bench so that the light is directed towards the wall on the far side of the room. The distance to the other side of the room is very large compared to the focal length, so this can be considered to be essentially infinity. Step 5: Turn on the light source, and slide the lens holder back and forth until a sharp image is formed on the opposite wall. When you have a sharp image formed far away, the object must be located at the focal point. Thus the distance from the lens to the F-diaphragm should be just the focal length. Measure this distance with the meter stick, and record it as the measured focal length below. Also record the focal length marked on the lens. Measured focal length cm. Marked focal length cm Question 1.2 Does your measured focal length agree with the value marked on the lens. PHYS-204: Physics II Laboratory 3
Lenses and Images v 0.1 Step 6: Replace the 15 cm lens with the 30 cm lens. Again adjust the position of the lens to get a clear image on the opposite wall. Measure the distance from the lens to the F-diaphragm, and record it, together with the value of the focal length marked on the lens. Measured focal length cm. Marked focal length cm Question 1.3 Does your measured focal length agree with the value marked on the lens. Image Object f f Figure 4: Many light rays converging at one point to form an image. Activity 1.2: Locating an Image Using a Ray Diagram We will now examine what determines the location of an image formed by the lens when the object is not at the focal point. Recall from last week’s lab how an image is formed. The lens deflects the rays of light that come from one point in the object so that they converge and all cross at some point. That point is where the image is formed, as in Fig. 4. Out of all these rays, there are three whose paths are easy to determine. These are shown in Fig. 5. To locate these rays it is only necessary to know the locations of the object, lens, and focal points. You should do this activity before coming to the lab, as part of the pre-lab assignment. Fig. 6 is a scale drawing of a lens with a focal length f = 15cm with an object located a distance do = 20cm to the left of the lens. The circles mark the locations of the focal points, 15cm to either side of the lens. In the next activity you will set up an object 20cm from a 15cm focal length lens, and locate the position at which the image is formed. Step 1: Use a ruler with a centimeter scale to measure the distance between from one focal point to the other in Fig. 6 of your lab handout. Using the fact the these two points represent points which are actually 30cm apart, work out the scale of the drawing - that PHYS-204: Physics II Laboratory 4
Page 1 and 2: Name: Lab Partners: Date: Pre-Lab A
Page 3: Lenses and Images v 0.1 Focal Point
Page 7 and 8: Lenses and Images v 0.1 Object f f
Page 9 and 10: Lenses and Images v 0.1 Object f f
Page 11 and 12: Lenses and Images v 0.1 behind the

Lab & Pre-lab #11

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