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 Aim: to Verify the Lens Maker's Equation, using a Converging Lens See The Focal Length of a Lens, Refractive Index, The Lens Equation The lens maker's equation is useful to... guess who... people who make lenses. The equation allows us to predict the focal length of a lens knowing the radii of curvature of its surfaces , r1 and r2 and the refractive index of the lens material, n. For a thin lens, surrounded by air, the equation is At first sight thus might seem to give a surprising result for any symmetrical lens (having the same curvature on both faces), namely, f = infinity. However, the equation is derived using a sign convention which can be summarized as follows. Imagine a beam of light passing through the lens; if it meets a convex surface, r has a positive value and if the light meets a concave surface, r is negative. From this we see that for a symmetrical lens, the equation can be written Method It is assumed that you can get hold of some sort of optical bench, something like that shown below. Also, since you will need to measure the radius of curvature of the lens surfaces, it is convenient to use a lens which is symmetrical (both surfaces the same curvature). Measure the image distances, v corresponding to as wide a range of object distances, u as possible. Plot a graph of 1/v against 1/u and use it to find the focal length of the lens. The next diagram should give you an idea of how to find the radius of curvature of the lens surfaces.

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