# A convex meniscus lens is made from glass with refractive index n = 1.52. If the radius of curvature of the convex surface is 20 cm and that of the concave surface is 40 cm, then find out the focal length.

given, for convex part, radius of curvature, R1=20 cm

and for concave part, R2=-40 cm

and refractive index, µ=1.52

$\begin{array}{l} 1 / f=(n-1)(1 / R 1+1 / R 2) \\ 1 / f=(1.52-1)(1 / 20-1 / 40) \\ 1 / f=0.52 \times(1 / 40) \\ f=76.92 \mathrm{cm} \end{array}$

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