Question

In the given figure, the radius of curvature of curved surface for both the plano-convex and plano-concave lens is 10 cm and refractive index for both is 1.5. The location of the final image after all the refractions through lenses is
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A
15 cm
B
20 cm
C
25 cm
D
40 cm
Solution
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The correct option is B 20 cm
The focal length of the plano - convex lens is:
1f1=(1.51)(1+101)=120
Focal length of plano - concave lens is:
1f2=(1.51)(1110)=120
since parallel beams are incident on the lens, its image from plano - concave lens will be formed at +20 cm from it (at the focus) and will act as an object for the plano - concave lens. since the two lens are at a distance of 10 cm from each other, therefore, for the next lens
u=+10 cm
v=ufu+f=10×201020
v=(20010) cm=20 cm
Hence, the correct option is (B)

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