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Q.

Two planoconvex lenses made glass of refractive index 1.5 and each having a focal length of 40 cm are arranged with their
curved surfaces touching each other. The space between them is now filled with water of refractive index 4/3. The focal length of the water lens formed is 

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a

- 120 cm

b

- 30 cm

c

- 60 cm

d

- 15 cm

answer is A.

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Detailed Solution

Plano convex 1f=μg−1×1R
140=0.5×1RR=20 cm
Focal length of water lens
1f=μL−1×−2R1f=13×−220⇒f=−30

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