A lens having refractive index 1.6 has focal length of 12 cm , when it is in air. Find the focal length of the lens when it is placed in water.
(Take refractive index of water as 1.28)

Question

A lens having refractive index 1.6 has focal length of 12 cm , when it is in air. Find the focal length of the lens when it is placed in water. (Take refractive index of water as 1.28)

TestHub · Accepted Answer

As we know, f 1 ​ = [ μ m ​ μ L ​ ​ − 1 ] [ R 1 ​ 1 ​ − R 2 ​ 1 ​ ] For air μ m ​ = 1 ​ 12 1 ​ = [ 1.6 − 1 ] [ R 1 ​ 1 ​ − R 2 ​ 1 ​ ] 12 1 ​ = 10 6 ​ [ R 1 ​ 1 ​ − R 2 ​ 1 ​ ] [ R 1 ​ 1 ​ − R 2 ​ 1 ​ ] = 72 10 ​ ​ For water f 1 ​ = [ 1.28 1.6 ​ − 1 ] [ 72 10 ​ ] = 128 32 ​ × 72 10 ​ ​ f 1 ​ = 4 1 ​ × 72 10 ​ f = 28.8 cm f = 288 mm ​

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