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Unformatted text preview: Compound Optics Prelab 1 Astronomical Telescope An astronomical telescope, intended for visual observation, is constructed with an objective lens with focal length f 1 = +150 mm and an eyepiece lens with focal length f 2 = +75 mm . The telescope is used for viewing distant objects ( s 1 >> f 1 ). Make a scale drawing of the telescope, showing the position of the intermediate image. What is the magnification of the telescope? The astronomical telescope is a system consisting of two lenses. The way to make calculations is to divide this system into two smaller subsystems, each consisting of one lens. The first system consists of the object, the objective lens, and the intermediate image, which is the image produced by the objective lens. The object is at a distance s 1 from the objective lens. The objec- tive lens has focal length f 1 . The distance between the objective lens and the (real) intermediate image is s 1 . (Since the intermediate image is real, do you expect it to be on the same side of the objective lens as the object, or do you expect it to be on the other side of the lens?) The second system consists of the intermediate image, which acts as the object for the eyepiece lens, the eyepiece lens, and the final image, viewed by the human eye. The distance between the intermediate image and the eyepiece lens is s 2 . The focal length of the eyepiece lens is f 2 . The distance between the eyepiece lens and the (virtual) final image is s 2 . If you look at the diagram of the telescope in your lab manual, you will see that the total distance between the objective lens and the eyepiece lens is the distance from the objective lens to the inter- mediate image plus the distance from the intermediate image to the eyepiece lens. In other words, D = s 1 + s 2 . In order to make a scale drawing of the telescope, we need to find the two distances s 1 and s 2 . In order to find these two distances, we will have to make some assumptions about s 1 and s 2 . 1.1 Intermediate Image The focal length of the objective lens is f 1 . We know that f 1 = 150 mm Let s 1 be the distance between the object and the objective lens. We are told that the telescope is used to view distant objects . This means that the distance s 1 is very, very large, s 1 Define the distance from the objective lens to the intermediate image to be s 1 . We need to find s 1 . Since we know the focal length f 1 and the distance s 1 , we can use the lens equation to find s 1 . The lens equation is 1 s 1 + 1 s 1 = 1 f 1 1 So 1 s 1 = 1 f 1- 1 s 1 We have made an assumption about the object distance s 1 . Use this assumption to calculate the inverse of the object distance, 1 s 1 Remember from your calculus class that lim x 1 x The lens equation should simplify to include just two terms. Now it is easy to take the inverse of the lens equation....
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- Fall '09