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 A
15 cm
B
20 cm
C
25 cm
D
40 cm
Solution
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#### The correct option is B 20 cmThe focal length of the plano - convex lens is: 1f1=(1.5−1)(1+10−1∞)=120Focal length of plano - concave lens is:1f2=(1.5−1)(1∞−110)=−120since 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 lensu=+10 cmv=ufu+f=10×2010−20⇒v=(20010) cm=20 cmHence, the correct option is (B)

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