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JAMB Physics 2006 Question 33

Question 33

Lenses and Optical Instruments

A person can focus objects when they lie beyond 75cm from his eyes. The focal length of the lens required to reduce his least distance of distinct vision to 25 cm is

  1. A.25.00 cm.
  2. B.37.50 cm.
  3. C.75.00 cm.
  4. D.18.75 cm.
Show the answer and explanation

Answer: B (37.50 cm.)

Explanation

This long-sighted person needs a converging lens that makes an object at u = 25 cm appear at 75 cm (virtual image, v = −75 cm). 1/f = 1/25 − 1/75 = 2/75, so f = 37.5 cm.