Q141 / 160 · TS EAMCET 2023 (14 May Shift 1) · Physics
Two convex lenses of focal lengths $20 \mathrm{~cm}$ and $30 \mathrm{~cm}$ are placed in contact with each other co-axially. The focal length of the combination is
- A$60 \mathrm{~cm}$
- B$10 \mathrm{~cm}$
- Correct answerC$12 \mathrm{~cm}$
- D$43 \mathrm{~cm}$
Solution
$\mathrm{f}_1=20 \mathrm{~cm} ; \mathrm{f}_2=30 \mathrm{~cm}$
The focal length of the combination is
$$
\begin{aligned}
& \frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{20}+\frac{1}{30}=\frac{3+2}{60}=\frac{5}{60}=\frac{1}{12} \\
& F=12 \mathrm{~cm}
\end{aligned}
$$
Hence, the correct answer is option: 'C'.