CDS 2026 (I) General Knowledge, question 16

Topic: Physics

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Q16

An object is placed at a distance of 15 cm in front of a concave mirror with a radius of curvature of 20 cm. Which one among the following statements is correct for the image formed ?

  1. (a) The image is 30 cm from the mirror on the same side as the object.Correct answer
  2. (b) The image is formed at a distance of 30 cm behind the mirror.
  3. (c) The image is 60 cm from the mirror on the same side as the object.
  4. (d) The image is formed at a distance of 60 cm behind the mirror.

Solution.

Answer: (a) The image is 30 cm from the mirror on the same side as the object.

  1. The focal length is half the radius of curvature: 10 cm.
  2. Using the Cartesian mirror convention, the object distance and focal length are both negative.
  3. The mirror equation gives the image distance as −30 cm, so the image is real and forms on the object’s side.
  4. f=−R/2=−10 cm,1v=1−10−1−15=−130,v=−30 cmf = - R/2 = - 10\, cm,\quad\frac{1}{v} = \frac{1}{- 10} - \frac{1}{- 15} = - \frac{1}{30},\quad v = - 30\, cm