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Focus and focal length - class-XII

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Multiple Choice

The radii of curvature of the surfaces of a double convex lens are 20 cm and 40 cm respectively, and its focal length is 20 cm. What is the refractive index of the material of the lens.? 

  1. $\dfrac{5}{2}$
  2. $\dfrac{4}{3}$
  3. $\dfrac{5}{3}$
  4. $\dfrac{4}{5}$
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