TS EAMCET · Physics · Ray Optics
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 \(10 \mathrm{~cm}\)
- C \(12 \mathrm{~cm}\)
- D \(43 \mathrm{~cm}\)
Answer & Solution
Correct Answer
(C) \(12 \mathrm{~cm}\)
Step-by-step Solution
Detailed explanation
\(\mathrm{f}_1=20 \mathrm{~cm} ; \mathrm{f}_2=30 \mathrm{~cm}\) The focal length of the combination is \[ \begin{aligned} & \frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{20}+\frac{1}{30}=\frac{3+2}{60}=\frac{5}{60}=\frac{1}{12} \\ & F=12 \mathrm{~cm} \end{aligned} \]
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