Q101 / 120 · JEE Main 2007 · Physics
Two lenses of power $-15 \mathrm{~D}$ and $+5 \mathrm{~D}$ are in contact with each other. The focal length of the combination is
- A$-20 \mathrm{~cm}$
- Correct answerB$-10 \mathrm{~cm}$
- C$+20 \mathrm{~cm}$
- D$+10 \mathrm{~cm}$
Solution
$\begin{aligned}
& P=P_1+P_2=-10 \\
& f=\frac{1}{P} \Rightarrow-0.1 \mathrm{~m} \Rightarrow-10 \mathrm{~cm}
\end{aligned}$
Hence, the correct answer is option: 'B'.