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NDA 2022 GAT · General Knowledge · Light and Optics · easy

A concave mirror has a radius of curvature of 40 cm. Its focal length is

Single correct option

A concave mirror has a radius of curvature of 40 cm. Its focal length is
  1. A. 40 cm
  2. B. 20 cm
  3. C. 10 cm
  4. D. 80 cm
Show answer and solution

Answer: (B) 20 cm

For a spherical mirror f = R/2 = 40/2 = 20 cm. The focus lies midway between the pole and the centre of curvature. Choosing 40 cm confuses the radius of curvature with the focal length, and 80 cm doubles it wrongly.
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Original question written in the NDA 2022 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