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Question:

A convex lens of refractive index 3/2 has a power of 2.5 D in air. If it is placed in a liquid of refractive index 2, then the new power of the lens is:- 1.25 D, - 1.5 D, 1.25 D, 1.5 D

1.25 D

1.5 D

  • 1.25 D

  • 1.5 D

Solution:

According to Lens Maker's Formula,Power of lens=P=(μr−1)(1R1−1R2)where μr is the refractive index of material of lens relative to the surrounding medium=μmaterialμmediumFor convex lens in air,μr=3/21=32For convex lens in the liquid,μr=3/22=34ThusP2P1=μr2−1μr1−1=34−132−1=−12⇒P2=−12×2.5D=−1.25D