A biconvex lens has two different refractive indices in different regions. The upper half has a refractive index 1.7 and the lower half has a refractive index 1.5 . The radius of curvatures of the two surfaces are R₁=15 ~cm and R₂=-25 ~cm . Image of a distant object will be formed at:

Step-by-step solution

The focal length f of a lens is given by the lens maker formula: 1 f = ( - 1) ( 1 R₁ - 1 R₂ ) . For the upper half, the refractive index ₁ = 1.7 . Given R₁ = 15 cm and R₂ = -25 cm , the focal length f₁ is calculated as: 1 f₁ = (1.7 - 1) ( 1 15 - 1 -25 ) = 0.7 ( 1 15 + 1 25 ) = 0.7 ( 5 + 3 75 ) = 0.7 8 75 = 5.6 75 . f₁ = 75 5.6 13.39 cm . For the lower half, the refractive index ₂ = 1.5 . The focal length f₂ is calculated as: 1 f₂ = (1.5 - 1) ( 1 15 - 1 -25 ) = 0.5 ( 1 15 + 1 25 ) = 0.5 ( 8 75 ) = 4 75 . f₂ = 75 4 =