Q 12-09-237JEE MainJEE Main 2025 (7 Apr, Shift 1)Easy
Two thin convex lenses of focal lengths $30\ \text{cm}$ and $10\ \text{cm}$ are placed coaxially, $10\ \text{cm}$ apart. The power of this combination is:
Answer: (D) $10\ \text{D}$
For two thin lenses separated by $d$:
$$\frac1F = \frac1{f_1} + \frac1{f_2} - \frac{d}{f_1f_2} = \frac1{30} + \frac1{10} - \frac{10}{300} = \frac1{10}\ \text{cm}^{-1}$$
$F = 10\ \text{cm} = 0.1\ \text{m}$, so $P = 10\ \text{D}$.
Solution by Sreeraj P, M.Sc Physics