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Q.

As shown in figure, a convergent lens is placed inside a cell filled with liquid. The lens has focal length + 20 cm when in air and its material has refractive index 1.50. If the liquid has refractive index 1.60, the focal length of the system is

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a

+80 cm

b

- 80 cm

c

-24 cm

d

-100 cm

answer is D.

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Detailed Solution

$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}}) H e r e, \frac{1}{f_{1}} = (1.5 - 1) (\frac{1}{20} + \frac{1}{20}) = \frac{0.5}{10} a n d \frac{1}{f_{2}} = - 0.6 (\frac{1}{20} + \frac{1}{20}) = - \frac{0.6}{10} \frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = - \frac{0.1}{10} = - \frac{1}{100} f = - 100 c m$
Hence the correct answer is option (b).

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