Optical Instruments & the Eye question from JEE Main 2026 (5 Apr, Shift 1)

Answer: (a)

1. Lens maker: biconvex $\dfrac{1}{f} = (\mu - 1)\dfrac{2}{R}$; plano-convex $\dfrac{1}{f'} = (\mu - 1)\dfrac{1}{R}$, so $f' = 2f_o$.
2. Normal adjustment: $M = \dfrac{L}{f_o} \cdot \dfrac{D}{f_e}$.
3. For the same $M$ with $f_o \to 2f_o$, $L$ must become $2L$.
4. So the answer is (a) increased two times.