A convex lens of refractive index 1.5 and focal length 18cm in air is… | JEE Main 2023 (24 Jan, Shift 2)

Q51JEE Main 2023 · 24 JAN · SHIFT 2Easy

Spherical Surfaces & Thin Lenses

A convex lens of refractive index 1.5 and focal length 18cm in air is immersed in water. The change in focal length of the lens will be ___________ cm.

(Given refractive index of water $= \frac{4}{3}$ )

Numerical answer type. Work it out, then open the answer.

Show answer and solution

Answer: 54

$\frac{f_w}{f_a} = \frac{\mu - 1}{\frac{\mu}{\mu_w} - 1} = \frac{0.5}{\frac{1.5}{4/3} - 1} = \frac{0.5}{0.125} = 4$
$f_w = 4 \times 18 = 72\text{ cm}$
Change $= 72 - 18 = 54\text{ cm}$

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