Given a converging lens of unknown focal length and unknown index of refraction, explain what materials you would need and what procedure you would follow in order to experimentally determine the focal length of the lens.
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OpenStax College Physics for AP® Courses, Chapter 25, Problem 16 (Test Prep for AP® Courses)

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This is College Physics Answers with Shaun Dychko. We have a converging lens and we don't know its focal length or its index of refraction and the question is asking what materials we need and what procedures should we follow to figure out the focal length of a lens? We will need some clamps to hold the lens and then we'll need an object to create an image of we'll need a ruler to measure the distance between the object and the lens and the distance between the lens and the screen— the screen's another thing we'll need— this screen-lens distance will be the image distance and the lens object distance will be the object distance and the procedure is to position this object until finally there's an image of it on the screen and when there is that's when these measurements are made and step (3) is to plug those numbers for the object distance and the image distance into this thin lens equation to figure out the focal length.