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Question

# A glass convex lens has a focal length of 20 cm in air. What will be its focal length, when it is completely immersed in a liquid of refractive index 1.63 ? ( Given aμg=1.5)

A
f1=127.4cm
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B
f1=125.4cm
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C
f1=120.4cm
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D
f1=18.75cm
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Solution

## The correct option is D f1=18.75cmfair =20 mμg=1.5μL=1.63 Using Lens formula 1fair =(μg−1)(1R1−1R2) 120=(0.5)(1R1−1R2)−(i) No lens immersed in liquid 1fliq=[aμ1aμg−1][1R1−1R2] 1fliq=(1.631.5−1)(1R1−1R2)−(ii) Dividing i/(ii) fliq20=0.5(1.63−1.55)×1.5fliq=0.5×1.5×200.8=18.75 cm

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