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Unformatted text preview: 1 Experiment #2: Telescopes And Microscopes Purpose: To measure the focal lengths of converging and diverging lenses and use this information to construct an astronomical telescope, a Galilean telescope, and a microscope. Equipment: Optical bench and pin accessory kit Two converging and one diverging lens (see note below) Light source Transparent ruler Erasable fine-tipped transparency felt pen (which can draw on the lenses) NOTE ABOUT LENSES—One converging lenses should have focal length in the 5-10 cm range and the other in the 10-15 cm range. A quick way of estimating this is to image a ceiling light onto a table and measure the distance between the lens and table. The diverging lens should have a focal length intermediate between the two converging lenses; in a darkened room you would then be able to form the image of a distant light with the combination of the diverging lens and the stronger converging lens. If you are unable to do this, i.e., if the diverging lens is stronger than both converging lenses, then you will need to mount all 3 lenses together in Step 3 of Part 1 and instead use the equation 1/ƒ 1 + 1/ƒ 2 +1/ƒ 3 =1/F in Step 4 of Part 1. On the other hand, if the diverging lens is weaker than both converging lenses, combine two diverging lenses together. Part 1—Focal Lengths of the Lenses Discussion: A converging lens is thicker at its center than at the periphery. Parallel incident rays pass through the lens and converge to a real focus on the opposite side. A diverging lens is thinner at the center than at the periphery. Parallel incident rays pass through the lens and diverge on the opposite side. These rays appear to originate from a virtual focus on the same side of the lens as the incident rays. The principal axis of a lens is a line drawn through its center perpendicular to the plane of the lens. The principal focal point F of a converging lens is that point on the principal axis where all incident rays parallel to the principal axis intersect after passing through the lens; for a diverging lens the principal focal point F is the point on the principal axis from where the diverging rays appear to originate from. There is also a secondary focal point F' which in both cases is located as far from the lens as the principal focal point is, but on the opposite side of the lens. The focal plane of a lens is the plane perpendicular to the principal axis which contains the principal focal point. 2 The focal length f of a lens is the distance along the principal axis from the center of the lens to its principal focal point. The power of a lens, measured in diopters, is the reciprocal of the focal length in meters, i.e., D ¡ 1/ f ....
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This note was uploaded on 08/05/2009 for the course PHY 2049 taught by Professor Ujj during the Spring '09 term at W. Florida.
- Spring '09