an air lens of radii of curvature 10 cm
(R1 = R2 = 10 cm) is cut in a cylinder of glass
refractive index (μ )=3/2.
The focal length and the nature of the lens is
(1) 15 cm, concave
(2) 15 cm, convex
(3) ∞, neither concave nor convex
(4) 0 cm, concave

1F=μairμg-11R1+1R2     =11.5-1110+110      =-0.51.5×210      =-115F=-15 cmHence,focal length = 15 cmNature of the lens: concave (due to negative sign of F)

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