A plano-convex lens, when silvered at its plane surface is equivalent to a concave mirror of focal length 28cm. When its curved surface is silvered and the plane surface not silvered, it is equivalent to a concave mirror of focal length 10cm, then the refractive index of the material of the lens is:
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a
914
b
149
c
179
d
None
answer is B.
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Detailed Solution
When plane surface is silvered, we use Peq=2PL=2μ−1R Then in case when curved surface is silvered, we use Peq=2PL+Pm , ⇒Peq=2μ−1R+2R=2μR Given that, feq=28 cmSo we use 28=R2μ−1....1When curved surface is silvered, we use feq=R2μ=10....2 From equation (1) and (2) we get, μ=149