Radius of curvature of the convex surface of a concavo-convex lens is 30 cm and that of the concave surface is 20 cm. The refractive index of glass is 1.5 , the focal length of the lens is

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-60 cm

-120 cm

120 cm

60 cm

answer is C.

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Detailed Solution

$R_{2} = + 30 cm R_{1} = + 20 cm$
$∴ \frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}}) \Rightarrow \frac{1}{f} = (1.5 - 1) [\frac{1}{(+ 20)} - \frac{1}{(+ 30)}] \Rightarrow f = + 120 cm .$

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