Lab & Pre-lab #11

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Name: Lab Partners: Date: Lenses and Images Objectives • To understand what is meant by the focal point of a lens. • To understand the relationship between the object distance, the image distance and the focal length of a lens. • To understand the types of images formed by a lens. Overview Lenses are used in many devices that we use everyday-they are essential for the operation of cameras and projectors. Lenses are needed for devices such as microscopes to magnify small objects so that we can see them. In the previous lab you studied the refraction of light rays by lenses. In this experiment we will continue this investigation by observing images formed by lenses in various arrangements, and determining how the positions and sizes of these images are related to the positions and sizes of the object. These labs have been adapted from the Real Time Physics Active Learning Laboratories [1]. The goals, guiding principles and procedures of these labs closely parallel the implementations found in the work of those authors [1, 2, 3]. Investigation 1: Images formed by a convex lens Every lens has two focal points. The importance of these points is that all light rays reaching the lens coming from a very distant object (ideally, infinitely far away) will be focused at a focal point, as in Fig. 1. The focal length is the distance from the center of the lens to a focal point. The focal length depends on how the lens is made. The curvature of the surfaces of the lens determines the focal length. Prediction 1.1 Will a lens whose surfaces have very little curvature have a smaller or larger focal length than one for which the surfaces have greater curvature To carry out this activity, you will need the following: • One 15 cm focal length convex lens. • One 30 cm focal length convex lens. • optical bench. PHYS-204: Physics II Laboratory 1
Lenses and Images v 0.1 Focal Point f f Focal Length Figure 1: Parallel light rays converge at the focal point. • light source. • F-diaphragm. • lens holders. • white projection screen. • meter stick. Activity 1.1: Focal Lengths of Convex Lenses Step 1: Examine the two convex lenses. The lenses have their focal length marked on them. One should have a focal length of +15 cm, the other +30 cm. Note which has the more sharply curved surfaces.. Question 1.1 To make a lens with shorter focal length, should the surfaces have greater or less curvature than for a longer focal length lens Which lens would you say a lens is more “powerful” if it has a shorter focal length, or a longer focal length Light rays which start from an object located at the focal point follow the paths shown in Fig. 1, but in the reverse direction. These rays diverge from the focal point, enter the lens and come out of the lens parallel to each other. This situation is depicted in Fig. 2. Step 2: Mount the light source and F-diaphragm at one end of your optical bench. The F- diaphragm should be right up against the light source, as in Fig. 3. This F-diaphragm provides the ”object in each of our optical systems. PHYS-204: Physics II Laboratory 2
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Lab & Pre-lab #11

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