Search any question & find its solution
Question:
Answered & Verified by Expert
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
Options:
Solution:
1675 Upvotes
Verified Answer
The correct answer is:
$12 \mathrm{~cm}$
$\mathrm{f}_1=20 \mathrm{~cm} ; \mathrm{f}_2=30 \mathrm{~cm}$
The focal length of the combination is
$$
\begin{aligned}
& \frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{20}+\frac{1}{30}=\frac{3+2}{60}=\frac{5}{60}=\frac{1}{12} \\
& F=12 \mathrm{~cm}
\end{aligned}
$$
The focal length of the combination is
$$
\begin{aligned}
& \frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{20}+\frac{1}{30}=\frac{3+2}{60}=\frac{5}{60}=\frac{1}{12} \\
& F=12 \mathrm{~cm}
\end{aligned}
$$
Looking for more such questions to practice?
Download the MARKS App - The ultimate prep app for IIT JEE & NEET with chapter-wise PYQs, revision notes, formula sheets, custom tests & much more.