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In an experiment a convex lens of focal length 15 cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5 cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20 cm from the lens. The focal length of the convex mirror is-

Options

  1. A20 . 0   cm
  2. B30 . 5   cm
  3. C25 . 0   cm
  4. D27 . 5   cm

Correct answer

D. 27 . 5   cm

Step-by-step solution

Image by Lens - - 1 - 20 + 1 v = 1 + 15 V = + 60 Distance of Image from mirror = 60 - 5 = 55   cm Path retraces when that image in at center of curvature of the mirror. Hence R = 55   cm f m i r r o r = R 2 = 27.5   cm

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