JEE Main · 2021 · 16 Mar 2021 · Shift 2 · Geometrical Optics

For a thin lens of refractive index ${\mu }_l$ placed in a medium of refractive index ${\mu }_m$,

Here ${\mu }_l={\mu }_m=1.4$, so the first bracket is zero and $\frac{1}{f}=0$ whatever $R_1$ and $R_2$ may be. Physically, a ray crossing a surface at which the refractive index does not change is not deviated at all, so the lens leaves every incident ray undeflected. Its focal length is infinite.