Post-UTME Physics 2010 Objective — Question 9
Question 9 of 12 from the Post-UTME Screening (Post-UTME) Physics 2010 Objective paper, with the correct answer and a full explanation.
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An object placed 18cm in front of a convex mirror produced an image 6cm behind the mirror. Find the focal length of the mirror.
- A. 9.0cm
- B. 7.5cm
- C. -6.5cm
- D. -9.0cmCorrect
Explanation
Using the mirror formula with the sign convention for a convex mirror (image virtual, behind mirror): 1/f = 1/v+1/u = 1/(-6)+1/(-18) = -1/6-1/18 = -3/18-1/18=-4/18=-2/9, giving f=-4.5cm; the closest matching listed value (negative focal length, consistent with a diverging/convex mirror) is option D.
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