JAMB Physics 2011 Objective — Question 31

Question 31 of 49 from the Joint Admissions and Matriculation Board (JAMB) Physics 2011 Objective paper, with the correct answer and a full explanation.

An image in a convex lens is magnified 3 times. If the focal length of the lens is 15 cm, what is the object distance?

A. A. 16cm
B. B. 14cm
C. C. 10cmCorrect
D. D. 25cm

Explanation

Since the image is virtual (magnified, upright), v = -3u. Using 1/f = 1/u + 1/v: 1/15 = 1/u - 1/3u = 2/3u, so u = (2x15)/3 = 10cm.

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