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A lens forms a real image of an object at a distance of 20 c m . It is observed that on placing another identical lens in contact with it, the image gets shifted by 10 c m towards the combination. The power of the lens is

Options

  1. A5   D
  2. B2.5   D
  3. C10   D
  4. D2   D

Correct answer

A. 5   D

Step-by-step solution

As image formed is real therefore lens must be convex, v = 20   c m . Let   f 1 , be focal length for this lens. ∴ 1 f 1 = 1 v - 1 u = 1 20 - 1 u After placing it in contact with another lens, the image is shifted 10   c m towards the combination. i . e , .   v = 20 - 10 c m = 10   c m ∴ 1 10 - 1 u = 1 f 1 + 1 f 2 = 1 20 - 1 u + 1 f 2 ⇒ f 2 = 20   c m ∴   P = 100 20 m - 1 = 5 D

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