passwella.
Menu

JAMB Physics 1989 Question 28

Question 28

Lenses and Optical Instruments

A far sighted person cannot see objects that are less than 100 cm away. If this person wants to read a book at 25 cm, what type and focal length of lens does he need?

  1. A.convex, 20 cm
  2. B.concave, 20 cm
  3. C.convex, 33 cm
  4. D.concave, 33 cm
Show the answer and explanation

Answer: C (convex, 33 cm)

Explanation

The lens must form a virtual image at 100 cm of a book at 25 cm: 1/f = 1/25 − 1/100 = 3/100, so f ≈ 33 cm, converging (convex).