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JAMB Physics 1999 Question 22

Question 22

Light and Reflection

The inside portion of part of a hollow metal sphere of diameter 20 cm is polished. The portion will therefore form a

  1. A.concave mirror of focal length 5 cm
  2. B.concave mirror of focal length 10 cm
  3. C.convex mirror of focal length 5 cm
  4. D.convex mirror of focal length 20 cm.
Show the answer and explanation

Answer: A (concave mirror of focal length 5 cm)

Explanation

A polished inside surface curves inwards, so it is a concave mirror. Its radius of curvature is 10 cm, so f = r/2 = 5 cm.