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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 = 4 3 )

Correct answer

0

Step-by-step solution

From Lens Maker's formula, For air: 1 f a = μ g μ a - 1   1 R 1 - 1 R 2 For water: 1 f w = μ g μ w - 1   1 R 1 - 1 R 2 ∴         1 f a 1 f w = μ g μ a - 1 μ g μ w - 1 ⇒ f w f a = 1 . 5 - 1 1 . 5 × 3 4 - 1 ⇒ f w = 0 . 5 0 . 125 × 18 = 72   cm Thus, change in focal length of the lens is 72 - 18 = 54   cm

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