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NDA 2020 GAT · General Knowledge · Light and Optics · hard

A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 20 cm. The power of the combination is

Single correct option

A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 20 cm. The power of the combination is
  1. A. +5 D
  2. B. -5 D
  3. C. zero
  4. D. +10 D
Show answer and solution

Answer: (C) zero

Power P = 100/f (cm): the convex lens has +5 D and the concave lens has -5 D. For lenses in contact the powers add, so the total is zero and the combination behaves like a plane glass plate. The traps are to pick +5 D or -5 D by ignoring one lens.
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Original question written in the NDA 2020 GAT pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake