A convex lens of refractive index 1.5 and focal length 18 cm in air is immersed in water. The change in focal length of the lens will be ________ cm. (Given refractive index of water ) [2023]

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Solution

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

(Given refractive index of water $= \frac{4}{3}$ ) [2023]

Ans.
(54)

$\frac{1}{f_{H_{2} O}} = (\frac{μ_{g}}{μ_{H_{2} O}} - 1) (\frac{2}{R}) = \frac{1}{8} (\frac{2}{R}) = \frac{1}{(4 f_{air})}$

$\Rightarrow f_{H_{2} O} = 4 f_{air} = 72 cm$

$So change in focal length = 72 - 18 = 54 cm$

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