The radius of curvature of the curved surface of a plano-convex lens is 20 cm. If the refractive index of the material of the lens be 1.5, then focal length of lens will be:

40 cm

The correct answer is Option (4) → 40 cm

The focal length of a plano-convex lens is,

$\frac{1}{f}=(μ-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

$=(1.5-1)\left(\frac{1}{20}-\frac{1}{∞}\right)$

$=\frac{0.5}{20}=\frac{1}{40}$

$=f=40cm$