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# AST101L6 - Astronomy 101 Online Telescopes Objectives When...

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Astronomy 101 Online - Telescopes Objectives : When you have completed this lab, you should be able to: · Determine how different lenses and mirrors will affect the paths of incident light rays. · Determine the properties of refracting and reflecting telescopes. Introduction : You will be studying different lenses and mirrors to determine their focal lengths. From what you learn, you will see how a simple refracting telescope is made and you will also be analyzing a simple reflecting telescope. Procedure : Part A : In this part of the experiment, we'll consider how parallel light rays interact with different lenses and mirrors. Click on the Picture 1 . What you will see is a simple box with a light source in it, designed to produce several rays of light. A plano-convex lens (one side is flat, the other side is curved) can be placed into the paths of the light rays to make them approximately parallel, as you can see in Picture 2 . We would like to see how different lenses and mirrors will interact with these parallel light rays. Click on Picture 3 . This type of lens is called a converging lens. 1 . Explain what happens to the parallel light rays after they strike the converging lens. Recall that the focal length is the distance from the lens or mirror to the point at which light rays are brought to a focus (to a sharp point). You'll notice that there is not a very sharp point here. Despite that, find the point at which the light rays are closest to being in focus. The convergence lens brings all light rays to an intersection exactly like a CRT big screen television. 2 . Focal length of the converging lens: _____Around the 6cm mark __________ Click on Picture 4 . This type of lens is called a diverging lens. 3 . Explain what happens to the parallel light rays after they strike the diverging lens. It does the exact opposite of a converging lens by scattering the light rays in different directions. As you can imagine, diverging lenses are typically not used in telescopes. However, the concave shape which makes up the sides of a diverging lens can be used to make a telescope mirror. Click on Picture 5 . Here you will see the parallel light rays interacting with a concave mirror.

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4 . Explain what happens to the parallel light rays after they strike the concave mirror. With the concave mirror it redirects the diverged light rays back to an intersecting point. Using the ruler shown in the picture, determine the focal length of the concave mirror. 5 . Focal length of concave mirror: ____2.5cm mark _________ Part B : Now that we've seen how converging lenses work, we'll use them to take some images of distant objects. Here is the setup . An optical bench is placed on a cart, positioned such that light from a distant object can pass through the lens on the optical bench and create an image on the screen. In the movies you are about to see, the object is a lamp.
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