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JEE Main 2020 (7 Jan, Shift 2) · Spherical Surfaces & Thin Lenses

Spherical Surfaces & Thin Lenses question from JEE Main 2020 (7 Jan, Shift 2)

Q86JEE Main 2020 · 7 JAN · SHIFT 2Medium

Spherical Surfaces & Thin Lenses

A thin lens made of glass (refractive index = 1.5) of focal length f = 16 cm is immersed in a liquid of refractive index 1.42. If its focal length in liquid is f1, then the ratio $\frac{f_1}{f}$ is closest to the integer:

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Answer: (c)

$\frac{f_1}{f} = \frac{\mu - 1}{\frac{\mu}{\mu_l} - 1} = \frac{0.5}{\frac{1.5}{1.42} - 1}$
$\frac{1.5}{1.42} - 1 = \frac{0.08}{1.42}$ , so $\frac{f_1}{f} = \frac{0.5 \times 1.42}{0.08} = 8.875$
So the answer is (c) 9.

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