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:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 40 cm
The focal length of a plano-convex lens is,
$\frac{1}{f}=(μ-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$
$=(1.5-1)\left(\frac{1}{20}-\frac{1}{∞}\right)$
$=\frac{0.5}{20}=\frac{1}{40}$
$=f=40cm$