Q71 / 90 · JEE Main 2021 (16 Mar Shift 2) · Physics

The refractive index of a converging lens is $1.4$ . What will be the focal length of this lens if it is placed in a medium of same refractive index ? (Assume the radii of curvature of the faces of lens are $R_{1}$ and $R_{2}$ respectively)

Solution

$\frac{1}{F} = [\frac{μ_{L}}{μ_{S}} - 1] [\frac{1}{R_{1}} - \frac{1}{R_{2}}]$

If $μ_{L} = μ_{S} \Rightarrow \frac{1}{F} = 0 \Rightarrow F = \infty$

Hence, the correct answer is option: 'B'.