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JAMB Physics 1979 Question 32

Question 32

Lenses and Optical Instruments

A concave lens of focal length 20 cm forms an image ½ the size of the object. The object distance is

  1. A.100 cm
  2. B.¹⁰⁰/₃ cm
  3. C.60 cm
  4. D.⁶⁰/₇ cm
  5. E.none of these value.
Show the answer and explanation

Answer: E (none of these value.)

Explanation

A concave lens forms a virtual image on the same side, so with f = −20 cm and v = −u/2: 1/u − 2/u = −1/20, giving u = 20 cm. That is not among A to D.