WAEC 2012 Physics · Question 18

An object is placed 10cm from a converging lens of foal length 15cm. Calculate the magnification of the image formed

Options

A.3.0
B.1.5
C.0.6
D.0.3

Answer: A. 3.0

Explanation written with AI and checked against the answer key

Analyze the question An object is placed 10 cm from a converging (convex) lens with focal length 15 cm. Since the object distance (u = 10 cm) is less than the focal length (f = 15 cm), the object is inside the focal length. This means the image will be virtual, upright, and magnified. Concept Lens formula: 1/f = 1/u + 1/v (real-is-positive convention) Magnification: m = v/u (magnitude) Solution 1/v = 1/f − 1/u 1/v = 1/15 − 1/10 1/v = (2 − 3)/30 = −1/30 v = −30 cm (negative means a virtual image on the same side as the object) Magnification: m = |v|/u = 30/10 = 3.0 Check with the direct formula: m = f/(f − u) = 15/(15 − 10) = 15/5 = 3.0 ✓ Answer The image is virtual, upright, and 3 times the size of the object. The correct option is A. 3.0. Tip When the object is placed between the lens and the focal point (u < f), the lens acts like a magnifying glass. Use the shortcut m = f/(f − u) to get the magnification quickly without finding v first. Also, if you get a negative v, don't panic. It simply means the image is virtual.

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