A convex lens of focal length f made of glass of refractive index 1.5 is immersed in water of refractive index 4/3. Then focal length:
Greater than f
−f
Less than f
f
A
f
B
−f
C
Greater than f
D
Less than f
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Solution
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Lensmaker equation gives the relation for focal length as
1f=(n1n2−1)Kshape
Given that, in air, the focal length of the lens is f
⇒Kshape=12f
Focal length in water is given by
1fwater=(n1n2−1)Kshape=(3/24/3−1)12f⇒fwater=16f
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