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For the thin convex lens, the radii of curvature are at 15 ~cm and 30 ~cm respectively. The focal length the lens is 20 ~cm . The refractive index of the material is :

Options

  1. A1.2
  2. B1.8
  3. C1.5
  4. D1.4

Correct answer

C. 1.5

Step-by-step solution

aligned & 1 f = ( _ lens _ air -1 ) ( 1 R ₁ - 1 R ₂ ) & 1 +20 = ( 1 -1 ) ( 1 +15 - 1 (-30) ) & 1 20 =( -1) ( 3 30 ) & -1= 1 2 & =1+ 1 2 = 3 2 =1 5 aligned

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