A bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. Refractive index n of the first lens is 1.5 and that of the second lens is 1.2 . Both the curved surface are of the same radius of curvature R=14 cm . For this bi-convex lens, for an object distance of 40 cm , the image distance will be

Step-by-step solution

The focal length (f₁ ) of the plano-cöneex Tens with n=1.5 using lens-maker formula 1 f₁ = (n₁-1 ) [ 1 R₁ - 1 R₂ ]=(1.5-1) [ 1 14 - 1 ]= 1 28 The focal length (f₂ ) of the plano-convex lens with n=1.2 1 f₂ = (n₂-1 ) [ 1 R₁ - 1 R₂ ]=(1.2-1) [ 1 - 1 -14 ]= 1 70 Focal length F of the combination 1 F = 1 f₁ + 1 f₂ = 1 20 Now, applying lens formula for the combination of lens array ll & 1 V - 1 U = 1 F 1 V - 1 -40 = 1 20 [ Given =40 ~cm ] & V=40 ~cm array