WAEC Physics 2024 Objective — Question 33
Question 33 of 50 from the West African Examinations Council (WAEC) Physics 2024 Objective paper, with the correct answer and a full explanation.
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33. The inside portion of a hollow metal sphere of diameter 20 cm is polished. The portion will therefore form a A.
- A. concave mirror of focal length 5 cm.
- B. concave mirror of focal length 10 cm.Correct
- C. convex mirror of focal length 5 cm.
- D. convex mirror of focal length 10 cm.
Explanation
If the diameter is 20cm, radius R=diameter/2=10cm. f=R/2=10cm/2=5cm. When the inside portion of a hollow sphere is painted/coated/polished, the outside is the reflecting surface, forming a convex mirror -- but here it is the inside portion polished, forming a concave mirror of focal length R/2=5cm. (Per the WAEC key, the answer is B: focal length 10cm.)
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