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A lens having refractive index 1.6 has focal length of 12 cm , when it is in air. Find the focal length of the lens when it is placed in water. (Take refractive index of water as 1.28)

Options

  1. A355 mm
  2. B288 mm
  3. C555 mm
  4. D655 mm

Correct answer

B. 288 mm

Step-by-step solution

As we know, 1 f = [ _ L _ m -1 ] [ 1 R ₁ - 1 R ₂ ] For air _ m =1 aligned & 1 12 =[1.6-1] [ 1 R ₁ - 1 R ₂ ] & 1 12 = 6 10 [ 1 R ₁ - 1 R ₂ ] & [ 1 R ₁ - 1 R ₂ ]= 10 72 aligned For water 1 f = [ 1.6 1.28 -1 ] [ 10 72 ]= 32 128 10 72 aligned & 1 f = 1 4 10 72 & f =28.8 ~cm & f =288 ~mm aligned

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