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Q.

A thin biconvex lens of focal length 20 cm is made of glass of refractive index 1.5. When it is dipped in a liquid of refractive index $\frac{9}{8}$ , it acts as

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a

Convex lens of focal length 30 cm

b

Concave lens of focal length 30 cm

c

Convex lens of focal length 15 cm

d

Concave lens of focal length 15 cm

answer is A.

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Detailed Solution

$\frac{\frac{1}{20}}{\frac{1}{f}} \frac{= (1.5 - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})}{= (\frac{3 / 2}{9 / 8} - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})}$
$\frac{f}{20} = \frac{\frac{1}{2}}{(\frac{3}{2} \frac{8}{9} - 1)} = \frac{1 / 2}{1 / 3} = \frac{3}{2}$
$f = 30 c m$

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A thin biconvex lens of focal length 20 cm is made of glass of refractive index 1.5. When it is dipped in a liquid of refractive index 98, it acts as