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
Answer & explanation
Correct answer: option 2
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.