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Assertion : A double convex lens ( =1.5) has focal length 10 ~cm . When the lens is immersed in water ( =4 / 3) , its focal length becomes 40 ~cm . Reason : 1 f = ( _g- _a _a ) ( 1 R₁ - 1 R₂ )

Options

  1. AIf both assertion and reason are true and reason is the correct explanation of assertion.
  2. BIf both assertion and reason are true but reason is not the correct explanation of assertion.
  3. CIf assertion is true but reason is false.
  4. DIf both assertion and reason are false.

Correct answer

A. If both assertion and reason are true and reason is the correct explanation of assertion.

Step-by-step solution

As 1 f = ( _g _a -1 ) ( 1 R₁ - 1 R₂ ) 1 10 = ( 3 2 -1 ) ( 1 R₁ - 1 R₂ ) ( 1 R₁ - 1 R₂ )= 1 5 If f_w is the focal length of the lens when immersed in water, 1 f_w = ( _g _w -1 ) ( 1 R₁ - 1 R₂ )= ( 3 / 2 4 / 3 -1 ) ( 1 5 )= 1 40 Hence, f_w=40 ~cm

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