the focal length of a convex lens is 18 cm andthe size of the image is a quarter of the object .the object is situated at a distance of
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Let the
object distance be x. Then the image distance would be (- x / n). The
focal length of the mirror is (- f). Using the mirror equation: 1/f =
1/Do + 1/Di, we obtain: - 1/f = 1/x - n/x → - 1/f = (1-n)/x → x = - f
(1-n). QEDCHECK: suppose f = 12 cm. n must be > 1; say it is 3. x = - 12 (-2) = 24 cm.Double check: 1/f = 1/24 - 1/8 = - 2 / 24 → f = - 12 cm (indicating that it is a convex mirror!)
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