Assertion : A double convex lens ( μ = 1.5 ) has focal length 10 cm . When the lens is immersed in water ( μ = 4/3 ) , its focal length becomes 40 cm . Reason : f 1 = ( μ a μ g − μ a ) ( R 1

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Assertion : A double convex lens ( μ = 1.5 ) has focal length 10 cm . When the lens is immersed in water ( μ = 4/3 ) , its focal length becomes 40 cm . Reason : f 1 ​ = ( μ a ​ μ g ​ − μ a ​ ​ ) ( R 1 ​ 1 ​ − R 2 ​ 1 ​ )

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As f 1 ​ = ( μ a ​ μ g ​ ​ − 1 ) ( R 1 ​ 1 ​ − R 2 ​ 1 ​ ) 10 1 ​ = ( 2 3 ​ − 1 ) ( R 1 ​ 1 ​ − R 2 ​ 1 ​ ) ⇒ ( R 1 ​ 1 ​ − R 2 ​ 1 ​ ) = 5 1 ​ If f w ​ is the focal length of the lens when immersed in water, f w ​ 1 ​ = ( μ w ​ μ g ​ ​ − 1 ) ( R 1 ​ 1 ​ − R 2 ​ 1 ​ ) = ( 4/3 3/2 ​ − 1 ) ( 5 1 ​ ) = 40 1 ​ Hence, f w ​ = 40 cm

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