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State PCS PYQPrelims 2025

BPSC – Combined Competitive Examination · Question 127

The focal length of a plane mirror is:

  1. +1 cm
  2. −1 cm
  3. 2 cm
  4. Infinity
Show answer & explanation

Answer: (D) Infinity

Explanation

A plane mirror can be seen as a sphere of infinite radius, so its focal length is infinite.

  • (A) Incorrect

    Wrong: a plane mirror has no finite focus.

  • (B) Incorrect

    Wrong: a plane mirror has no finite focus.

  • (C) Incorrect

    Wrong: a plane mirror has no finite focus.

  • (D) Correct

    Correct: its radius of curvature is infinite, so f = R/2 is infinite.