1 1 The formula 1 f do d + describes the focal length f of a lens, where do is the distance from the lens to the object and d, is the distance from the lens to the image. The image of a lighthouse, which is inverted as the light passes through the lens, is located 3m from the thin convex lens. The focal length f is an unknown distance between the lens and the image the image is 1.2m tall. The lighthouse is located 21m from the lens. 21 m (b) What distance is f from the lens? 3m (a) To the nearest tenth of a meter, how tall is the lighthouse? T 1.2 m I (e) To the nearest tenth of a meter, what is the image height when the image is located at the focal length?