A convex lens forms a real image 2 times the size of the object. The distance be

Question

A convex lens forms a real image 2 times the size of the object. The distance between object and image is 53 cm. Find the focal length of the lens. Answer in units of cm.

Explanation / Answer

Given that        hi = 2 ho do - di = 53 cm ==> do = 53 + di    f = ? We know that             hi / ho = - di / do        ==> 2 ho / ho = - di / 53 +di       ==> 2 ( 53 + di) = - di       == > 106 = -3di      ==> di = -106 / 3 = -35.33 cm so that       do = 53 - 35.33 = 17.66 cm We know that                     f = do * di / do +di                       = 17.66 * (-35.33) / (17.66 - 35.33)                       = 35.33 cm

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