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JEE Main202624 January 2026Evening ShiftPhysicsRay OpticsActual

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

Options

  1. A-15 / 2
  2. B-15
  3. C-10
  4. D-20

Correct answer

B. -15

Step-by-step solution

For a mirror with magnification m = -3 (real image), object and image distances satisfy |u| + |v| = 40 cm. Since m = v/u = -3, we have v = -3u, giving u + 3u = 40, so u = 10 cm and v = -30 cm. Using mirror formula: 1 f = 1 u + 1 v = 1 10 + 1 -30 = 3-1 30 = 1 15 Hence f = -15 cm.

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