In the figure, an air lens of radii of curvature 10 cm(R1=R2=10cm) is cut in a cylinder of glass (μ=1.5). The focal length and the nature of the lens is
A
15cm, concave
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B
15cm, convex
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C
∞, neither concave nor convex
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D
0, concave
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Solution
The correct option is C15cm, concave 1f=(gμa−1)(1R1−1R2)=(23−1)(210) ⇒f=−15cm, so behaves as concave lens.