Inside a vessel filled with liquid a converging lens is placed as shown in figure. The lens has focal length 15 cm when in air and has refractive index 3/2. If the liquid has refractive index 9/5, the focal length of lens in liquid is (a) 15 cm (b) - 60 cm (c) 90 cm (d) - 45 cm

Answer: (d). fl = fa (μ - 1)/((μ)/(μl) - 1) (μ)/(μl) = (3/2)/(9/5) = 5/6, so fl = 15 × (0.5)/(-1/6) = -45 cm The lens becomes diverging in the denser liquid. So the answer is (d) -45 cm.

Inside a vessel filled with liquid a converging lens is placed as… | MHT CET 2021 (20 Sep, Shift 2)

Q79MHT CET 2021 · 20 SEP · SHIFT 2Medium

Spherical Surfaces & Thin Lenses

Inside a vessel filled with liquid a converging lens is placed as shown in figure. The lens has focal length $15\text{ cm}$ when in air and has refractive index $\frac{3}{2}$ . If the liquid has refractive index $\frac{9}{5}$ , the focal length of lens in liquid is

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Answer: (d)

$f_l = f_a \frac{\mu - 1}{\frac{\mu}{\mu_l} - 1}$
$\frac{\mu}{\mu_l} = \frac{3/2}{9/5} = \frac{5}{6}$ , so $f_l = 15 \times \frac{0.5}{-1/6} = -45$ cm
The lens becomes diverging in the denser liquid.
So the answer is (d) $-45$ cm.

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Spherical Surfaces & Thin Lenses question from MHT CET 2021 (20 Sep, Shift 2)

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