Question

A lens has a convex surface one side and a concave surface another side. If the radius of curvature of convex and concave are equal to $R$ each and $\mu$ is refractive index of lens then its focal length is:

• A. $\frac{2R}{\mu -1}$
• B. $\frac{R}{2(\mu -1)}$
• C. $Zero$
• D. $Infinite$

Answer 1

The correct option is D $Infinite$

By lens maker formula.

$\frac{1}{f}=(\mu -1)(\frac{1}{R_{convex}}-\frac{1}{R_{concave}})$

$1/f=(\mu -1)(\frac{1}{R}-\frac{1}{R})$

$1/f=(\mu -1)\left(0\right)$

$\frac{1}{f}=0$

$\Rightarrow f=\infty$

So, focal length is infinite.