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
$\text{Focal length of lens in air is }\frac{1}{f} = (\mu_g - 1)(\frac{1}{R_1} - \frac{1}{R_2}) = (1.5 - 1)(\frac{1}{20} - \frac{1}{-20}) = 0.5 \times \frac{1}{10} $
$\Rightarrow f = 20cm$
Rays will converge at focal point i.e 20 cm from the optical centre of the lens.