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An object 50 cm tall is placed on the principal axis of a convex lens, its 20 cm tall image is formed on the screen placed at a distance of 10 cm from the lens. Calculate the focal length of the lens.

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The lens formula is 1/f = 1/v - 1/u. The magnification formula is -v/u = h'/h.

From the magnification formula, we get v = -20/50 * u. Substituting this into the lens formula gives us 1/f = -50/20 * 1/u - 1/u. Solving this equation for f will give you the focal length of the lens.
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