Both radii of curvature of a convex lens are 20 cm and refractive index of the material of the lens is 1.5. Rays parallel to the axis of the lens will converge at

10 cm 20 cm 30 cm 40 cm

20 cm

As $\frac{1}{f}=(μ-1)(\frac{1}{R_1}-\frac{1}{R_1})$ $∴\frac{1}{f}=(1.5-1)(\frac{1}{20}-\frac{1}{-20})=\frac{0.5×2}{20}=\frac{1}{20}$ or f = 20 cm i.e., rays will converge at 20 cm from the optical centre of the lens.