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astlblt - Laboratory 2 Introduction to Lenses Telescopes...

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10 Laboratory 2 - Introduction to Lenses & Telescopes Materials Used: A set of four lenses, an optical bench with a centimeter scale, a white screen, several lens holders, a light source (with crossed arrows), a set of high-intensity lamps (mounted in the front of the room), marking tape, a lens cleaning kit. Objectives: To understand basic optical system parameters; to relate image and object distances to focal lengths for various lenses; to determine the focal length, image size, brightness, and f - number for various lenses; to understand the differences between telescopic systems and astronomical telescopes; to construct a simple refracting telescope and to understand it’s performance. Discussion: First we must distinguish between a t e l e s c o p i c s y s t e m and an a s t r o n o m i c a l t e l e s c o p e . Though both are “telescopic” they are distinguishable. A telescopic system is anything that produces parallel rays of light. An a s t r o n o m i c a l t e l e s c o p e is, in addition to this, a light amplification system. Many objects of interest in the night sky are large enough but not bright enough to be easily viewed (the Andromeda Galaxy is a great example of this - being nearly the size of the full moon but dim and difficult to see). Although astronomical telescopes do, indeed, magnify objects, their main function is to make dim objects brighter. Most of the light rays that we see, unless they are coming from objects that are v e r y close, are approximately parallel to each other as they move through space (left in Figure 1). Optical systems create images by by f o c u s i n g rays of light – generally with lenses and/or mirrors. In order to achieve focus, rays of light they must be redirected so that instead of traveling parallel to each other they converge on a f o c a l p o i n t as shown in Figure 1. The focal point of an optical system is where images are formed. Focusing light rays is generally accomplished in one of two ways: by r e f r a c t i o n through curved glass lenses – which exploits the fact that light rays bend when traveling from one medium to the next, or by r e f l e c t i o n from curved mirrors – which exploits the fact that light rays reflect at the same angle as they impinge upon a reflecting surface. Although both arrangements are used to focus light we will concentrate on refractory methods of obtaining images. In this procedure we will explore a simple refracting telescope that employs three lenses to produce an image on the retinal plane of your eye. We’ll begin by discovering the properties of lenses and then determining how they may be arranged to form a refracting telescope. Focal point lens Figure 1. A refractory focusing arrangement.
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11 For the purpose of this exercise a c o n v e r g i n g l e n s is one with two convex surfaces such as the one shown in Figure 2. Our telescope will contain converging lenses.
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