JEE Main 2026 · 24 January · evening · Single correct · +4 / −1

Distance between an object and three times magnified real image is 40 cm . The focal length of the mirror used is ____ cm .

  1. (a) -20
  2. (b) -15/2
  3. (c) -10
  4. (d) -15
Answer and solution

Answer (d) -15

Solution

Physics sir · 3 steps

A real, magnified image from a mirror means concave, with object and image on the same side.

  1. Which mirror. Only a concave mirror gives a real, magnified image. A real image from a mirror is inverted, so m = -3, and m = -v/u gives v = 3u.
  2. Use the gap. A real image forms in front of the mirror, on the same side as the object. Take u = -x and v = -3x. The distance between them is 3x - x = 2x = 40 cm, so x = 20 cm. That gives u = -20 cm and v = -60 cm.

    1/f = 1/v + 1/u = -1/60 - 1/20 = -4/60 = -1/15

  3. So. f = -15 cm. The negative sign confirms a concave mirror, so the answer is (d).

The idea to keep. For a mirror, a real image lies on the same side as the object, so the object-image distance is |v| - |u|. Write v in terms of u using m, then apply the mirror formula with signs.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Ray Optics

Asked about 200 times in JEE Main since 2020. This question is from 2026.