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JAMB Physics 1986 Question 31

Question 31

Light and Reflection

A dentist obtains a linear magnification of 4 of a hole in a tooth by placing a concave mirror at a distance of 2.0 cm from the tooth. The radius of curvature of the mirror is

  1. A.5.3 cm
  2. B.3.2 cm
  3. C.2.7 cm
  4. D.1.6 cm
Show the answer and explanation

Answer: A (5.3 cm)

Explanation

The dentist sees an upright, virtual image, so v = −8 cm when u = 2 cm. 1/f = 1/2 − 1/8 = 3/8, f ≈ 2.67 cm and r = 2f ≈ 5.3 cm.