JAMB 2010 Physics · Question 36

An object 4 cm high is placed 15 cm from a concave mirror of focal length 5 cm. The size of the image is?

Options

A.2 cm
B.3 cm
C.4 cm
D.5 cm

Answer: A. 2 cm

Explanation written with AI and checked against the answer key

Analyze the question: A concave mirror has f = 5 cm. The object (h₀ = 4 cm) is at u = 15 cm. We need the image size. Concept: Use the mirror formula and the magnification formula: 1/f = 1/u + 1/v m = v/u = h_i/h₀ (ignoring signs for size) Solution: 1/v = 1/f − 1/u = 1/5 − 1/15 = 3/15 − 1/15 = 2/15 v = 15/2 = 7.5 cm (real image, in front of the mirror) Magnification: m = v/u = 7.5/15 = 0.5 Image height: h_i = m × h₀ = 0.5 × 4 = 2 cm The image is real, inverted and diminished, which makes sense because the object is beyond the centre of curvature (C = 2f = 10 cm, and 15 cm > 10 cm). Answer: The image is 2 cm high, which is option A. Tip: When the object is beyond C (u > 2f) for a concave mirror, the image is always smaller than the object, so you can quickly eliminate options that are equal to or larger than 4 cm (C and D). Then a quick calculation of m = f/(u − f) = 5/10 = 0.5 gives you the answer directly.

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