AP EAMCET · PHYSICS · Ray Optics
The dispersive powers of the materials of two lenses forming an achromatic combination are in the ratio of \(4: 3\). Effective focal length of the two lenses is \(+60 \mathrm{~cm}\) then the focal lengths of the lenses should be
- A \(-20 \mathrm{~cm}, 25 \mathrm{~cm}\)
- B \(20 \mathrm{~cm},-25 \mathrm{~cm}\)
- C \(-15 \mathrm{~cm}, 20 \mathrm{~cm}\)
- D \(15 \mathrm{~cm},-20 \mathrm{~cm}\)
Answer & Solution
Correct Answer
(D) \(15 \mathrm{~cm},-20 \mathrm{~cm}\)
Step-by-step Solution
Detailed explanation
\(\begin{aligned} P=P_1+P_2 & =\frac{q}{f_1}+\frac{q}{f_2} \\ \frac{1}{60} & =\frac{q}{f_1}+\frac{q}{f_2} \\ \frac{1}{60} & =\frac{f_1+f_2}{f_1 f_2}\end{aligned}\) According to question, \(\frac{\omega_1}{\omega_2}=\frac{f_1}{f_2} \Rightarrow \frac{f_1}{f_2}=\frac{-4}{3}\) or…
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