A plano-convex lens has diameter 5 cm and its thickness at the centre is 0 . 25 cm . If the speed of light inside the lens is 2 × 10 8 ms - 1 , then what is the focal length (in cm ) of the lens?

Question

A plano-convex lens has diameter 5 cm and its thickness at the centre is 0 . 25 cm . If the speed of light inside the lens is 2 × 10 8 ms - 1 , then what is the focal length (in cm ) of the lens?,

MARKS App · Accepted Answer

R. I. of lens, μ = c v = 3 × 10 8 2 × 10 8 = 3 2 From ΔACO , radius of curvature of lens is, R 2 = R 1 2 + ( R - 0 . 25 ) 2 ∴ R 2 = ( 2 . 5 ) 2 + ( R - 0 . 25 ) 2 ∴ R 2 = 6 . 25 + R 2 + 0 . 0625 - 0 . 5 R ∴ 0 . 5 R = 6 . 3125 ∴ R = 12 . 625 cm F = R μ - 1 = 12 . 625 3 2 - 1 = 25 . 25 cm

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