Question

A thin concavo-convex lens has two surfaces of radii of curvature R and 2R. The material of lens has a refractive index $\mu$. When kept in air the focal length of lens

• A. will depend on the direction from which light is incident on it
• B. will be the same irrespective of the direction from which light is incident on it
• C. will be equal to $\frac{R}{\mu -1}$
• D. will be equal to $\frac{2R}{\mu -1}$

Answer 1

Answer: (B), (D)

The correct options are
B will be the same irrespective of the direction from which light is incident on it
D will be equal to $\frac{2R}{\mu -1}$

$\frac{1}{f}=[\mu -1][\frac{1}{R}-\frac{1}{2R}]$

$\Rightarrow f=\frac{2R}{[\mu -1]}$

The focal length of the whole lens is dependent upon the refractive indexes and the dimensions of the lens and is the same irrespective of the direction from which light is incident on it