JAMB Physics 2020 Objective — Question 15

Question 15 of 40 from the Joint Admissions and Matriculation Board (JAMB) Physics 2020 Objective paper, with the correct answer and a full explanation.

15. A concave mirror of radius of curvature 20cm forms a real image four times as large as the object. The object distance is

A. A. 60cm
B. B. 40cm
C. C. 30cm
D. D. 12.5cmCorrect

Explanation

f=r/2=10cm. Since magnification M=v/u=4, v=4u. Using mirror formula 1/f=1/v+1/u: 1/10=1/4u+1/u=5/4u. 4u=50. u=12.5cm.

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