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TS EAMCET · Physics · Ray Optics

In Ramsden eyepiece, the two planoconvex lenses each of focal length \(f\) are separated by a distance \(12 \mathrm{~cm}\). The equivalent focal length (in \(\mathrm{cm}\) ) of the eyepiece is

  1. A \(10.5\)
  2. B \(12.0\)
  3. C \(13.5\)
  4. D \(15.5\)
Verified Solution

Answer & Solution

Correct Answer

(C) \(13.5\)

Step-by-step Solution

Detailed explanation

\(d=\frac{2 f}{3}\) \(f=\frac{3 d}{2}=\frac{3 \times 12}{2}=18 \mathrm{~cm}\) Equivalent focal length is \(\begin{aligned} f^{\prime} & =\frac{f_1 f_2}{f_1+f_2}+\frac{f}{4}=\frac{18 \times 18}{18+18}+\frac{18}{4} \\ & =9+4.5 \\ & =13.5 \mathrm{~cm}\end{aligned}\)